Diacylglycerol acyltransferases for alteration of polyunsaturated fatty acids and oil content in oleaginous organisms

ABSTRACT

Acyltransferases are provided, suitable for use in the manufacture of microbial oils enriched in omega fatty acids in oleaginous yeast (e.g.,  Yarrowia lipolytica ). Specifically, genes encoding diacylglycerol acyltransferase (DGAT1) have been isolated from  Y. lipolytica  and  Mortierella alpina . These genes encode enzymes that participate in the terminal step in oil biosynthesis in yeast. Each is expected to play a key role in altering the quantity of polyunsaturated fatty acids produced in oils of oleaginous yeasts.

This application claims the benefit of U.S. Provisional Application60/624,812, filed Nov. 4, 2004.

FIELD OF THE INVENTION

This invention is in the field of biotechnology. More specifically, thisinvention pertains to the identification of nucleic acid fragmentsencoding diacylglycerol acyltransferases (DGAT1) andacyl-CoA:sterol-acyltransferases. These enzymes are useful for alteringthe quantity of oil in oleaginous microorganisms, such as oleaginousyeasts.

BACKGROUND OF THE INVENTION

The present invention is directed toward the development of anoleaginous yeast that accumulates oils enriched in long-chain ω-3 and/orω-6 polyunsaturated fatty acids (“PUFAs”; e.g., 18:3, 18:4, 20:3, 20:4,20:5 and 22:6 fatty acids). Toward this end, the natural abilities ofoleaginous yeast (mostly limited to 18:2 fatty acid production) havebeen enhanced by advances in genetic engineering, leading to theproduction of 20:4 (arachidonic acid, or “ARA”), 20:5 (eicosapentaenoicacid, or “EPA”) and 22:6 (docosahexaenoic acid, or “DHA”) PUFAs intransformant Yarrowia lipolytica. These ω-3 and ω-6 fatty acids wereproduced by introducing and expressing heterologous genes encoding theω-3/ω-6 biosynthetic pathway in the oleaginous host (see co-pending U.S.patent application Ser. No. 10/840,579, entirely incorporated herein byreference). However, in addition to developing techniques to introducethe appropriate fatty acid desaturases and elongases into theseparticular host organisms, it is also necessary to increase the transferof PUFAs into storage lipid pools following their synthesis.

Most free fatty acids become esterified to coenzyme A (CoA), to yieldacyl-CoAs. These molecules are then substrates for glycerolipidsynthesis in the endoplasmic reticulum of the cell, where phosphatidicacid and diacylglycerol (DAG) are produced. Either of these metabolicintermediates may be directed to membrane phospholipids (e.g.,phosphatidylglycerol, phosphatidylethanolamine, phosphatidylcholine) orDAG may be directed to form triacylglycerols (TAGs), the primary storagereserve of lipids in eukaryotic cells.

Two comprehensive mini-reviews on TAG biosynthesis in yeast, includingdetails concerning the genes involved and the metabolic intermediatesthat lead to TAG synthesis are: D. Sorger and G. Daum, Appl. Microbiol.Biotechnol. 61:289-299 (2003); and H. Mullner and G. Daum, ActaBiochimica Polonica, 51 (2):323-347 (2004). However, the authorsacknowledge that most work performed thus far has focused on the yeastSaccharomyces cerevisiae and numerous questions regarding TAG formationand regulation remain.

Briefly, three pathways have been described for the synthesis of TAGs inS. cerevisiae (Sandager, L. et al., J. Biol. Chem. 277(8):6478-6482(2002)). First, TAGs are mainly synthesized from DAG and acyl-CoAs bythe activity of a diacylglycerol acyltransferase (i.e., DGAT2, encodedby the DGA1 gene). More recently, however, a phospholipid:diacylglycerolacyltransferase (i.e., PDAT, encoded by the LRO1 gene) has also beenidentified that is responsible for conversion of phospholipid and DAG tolysophospholipid and TAG, respectively, thus producing TAG via anacyl-CoA-independent mechanism (Dahlqvist et al., PNAS. 97(12):6487-6492(2000)). Finally, two acyl-CoA:sterol-acyltransferases (encoded by theARE1 and ARE2 genes) are known that utilize acyl-CoAs and sterols toproduce sterol esters (and TAGs in low quantities; see Sandager, L. etal., Biochem. Soc. Trans. 28(6):700-702 (2000)). Together, PDAT andDGAT2 are responsible for approximately 95% of oil biosynthesis in S.cerevisiae.

Although homologs of each of the acyltransferase genes described abovehave been identified in various other organisms and disclosed in thepublic literature, few genes are available from organisms classified asoleaginous. With respect to yeast, those species included within thegenera of Yarrowia, Candida, Rhodotorula, Rhodosporidium, Cryptococcus,Trichosporon and Lipomyces and that can accumulate at least 25% of theirdry cell weight as oil are classified as oleaginous. Within this uniquefamily of yeast, however, only two acyltransferases have been isolatedand characterized. These include a DGAT2 and PDAT from Yarrowialipolytica (see co-pending U.S. patent application Ser. No. 10/882,760,entirely incorporated herein by reference). However, in contrast to thefindings in Saccharomyces cerevisiae, the Y. lipolytica DGAT2 and PDATwere discovered to only be partially responsible for the organism'stotal oil biosynthesis.

There remains a need there for to identify genes encoding diacylglycerolacyltransferases (DGAT1) and acyl-CoA:sterol-acyltransferases useful forexpression in oleaginous yeast for the production of PFUA's. The presentwork was conducted to identify and characterize the additional gene(s)involved in oil biosynthesis in the oleaginous yeast, Yarrowialipolytica. An understanding of the native mechanisms of oilbiosynthesis in this organism is useful, prior to the development oftechniques that modify the transfer of recombinantly produced fattyacids (e.g., long-chain PUFAs, such as ARA, EPA and DHA) to the storagelipid pools (i.e., TAG fraction) within transformant oleaginous yeast.

Applicants have solved the stated problem by isolating the genesencoding a diacylglycerol acyltransferase (DGAT1) and anacyl-CoA:sterol-acyltransferase (ARE2) from the oleaginous yeast,Yarrowia lipolytica. Together, the PDAT, DGAT2 and DGAT1 of Yarrowialipolytica are responsible for up to ˜95% of oil biosynthesis (whileARE2 may additionally be a minor contributor to oil biosynthesis).Additionally, an orthologous DGAT1 gene was cloned from Mortierellaalpina (an oleaginous fungus) and four other fungal DGAT1 orthologs frompublic sequence databases (i.e., Neurospora crassa, Gibberella zeaePH-1, Magnaporthe grisea and Aspergillus nidulans) were identified. Withthese fungal DGAT1 protein sequences, the Applicants have discovereddiagnostic features that will be useful to identify subsequent geneswithin this family of proteins. These DGAT1 genes will be useful toenable one to modify the transfer of long-chain free fatty acids (e.g.,ω-3 and/or ω-6 fatty acids) to the TAG pool in oleaginous yeast.

SUMMARY OF THE INVENTION

The invention relates to the discovery of genes encoding acyltransferaseenzymes. The genes and encoded enzymes are useful in manipulating theproduction of commercially useful oils in microorganisms, andparticularly in oleaginous yeasts. Accordingly the invention provides anisolated nucleic acid molecule encoding a diacylglycerolacyltransferase-1 enzyme, selected from the group consisting of:

-   -   (a) an isolated nucleic acid molecule encoding the amino acid        sequence selected from the group consisting of SEQ ID NOs:14 and        18;    -   (b) an isolated nucleic acid molecule that hybridizes with (a)        under the following hybridization conditions: 0.1×SSC, 0.1% SDS,        65° C. and washed with 2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1%        SDS; or    -   (c) an isolated nucleic acid molecule that is completely        complementary to (a) or (b).

Similarly, the invention provides an isolated nucleic acid moleculeencoding an acyl-CoA:sterol-acyltransferase, selected from the groupconsisting of:

-   -   (a) an isolated nucleic acid molecule encoding the amino acid        sequence as set forth in SEQ ID NO:16;    -   (b) an isolated nucleic acid molecule that hybridizes with (a)        under the following hybridization conditions: 0.1×SSC, 0.1% SDS,        65° C. and washed with 2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1%        SDS; or    -   (c) an isolated nucleic acid molecule that is completely        complementary to (a) or (b).

Additionally the invention provides polypeptides encoded by the isolatednucleic acid molecules of the invention as well as genetic chimera andhost cells expressing the same.

In another embodiment the invention provides an isolated nucleic acidmolecule encoding an amino acid motif selected from the group consistingof:

-   -   a) SEQ ID NO:31;    -   b) SEQ ID NO:32;    -   c) SEQ ID NO:33;    -   d) SEQ ID NO:34;    -   e) SEQ ID NO:35;    -   f) SEQ ID NO:36;    -   g) SEQ ID NO:37;    -   h) SEQ ID NO:23;    -   i) SEQ ID NO:24;    -   j) SEQ ID NO:25;    -   k) SEQ ID NO:26;    -   l) SEQ ID NO:27;    -   m) SEQ ID NO:28;    -   n) SEQ ID NO:29; and    -   o) SEQ ID NO:30.

In another embodiment the invention provides an amino acid motifsequence selected from the group consisting of:

-   -   a) SEQ ID NO:31;    -   b) SEQ ID NO:32;    -   c) SEQ ID NO:33;    -   d) SEQ ID NO:34;    -   e) SEQ ID NO:35;    -   f) SEQ ID NO:36;    -   g) SEQ ID NO:37;    -   h) SEQ ID NO:23;    -   i) SEQ ID NO:24;    -   j) SEQ ID NO:25;    -   k) SEQ ID NO:26;    -   l) SEQ ID NO:27;    -   m) SEQ ID NO:28;    -   n) SEQ ID NO:29; and    -   o) SEQ ID NO:30.

In a preferred embodiment the invention provides a method of increasingtriacylglycerol content in a transformed host cell comprising:

-   -   (a) providing a transformed host cell comprising:        -   (i) at least one gene encoding a diacylglycerol            acyltransferase 1 enzyme having the amino acid sequence            selected from the group consisting of SEQ ID NOs:14, 18, 19,            20, 21 and 22 under the control of suitable regulatory            sequences; and,        -   (ii) a source of fatty acids;    -   (b) growing the cell of step (a) under conditions whereby the at        least one gene encoding a diacylglycerol acyltransferase 1        enzyme is expressed, resulting in the transfer of the fatty        acids to triacylglycerol; and    -   (c) optionally recovering the triacylglycerol of step (b).

In similar fashion the invention provides a method of increasing the ω-3or ω-6 fatty acid content of triacylglycerols in a transformed host cellcomprising:

-   -   (a) providing a transformed host cell comprising:        -   (i) genes encoding a functional ω-3/ω-6 fatty acid            biosynthetic pathway;        -   (ii) at least one gene encoding a diacylglycerol            acyltransferase 1 enzyme having the amino acid sequence            selected from the group consisting of SEQ ID NOs:14, 18, 19,            20, 21 and 22 under the control of suitable regulatory            sequences;    -   (b) growing the cell of step (a) under conditions whereby the        genes of (i) and (ii) are expressed, resulting in the production        of at least one ω-3 or ω-6 fatty acid and its transfer to        triacylglycerol; and    -   (c) optionally recovering the triacylglycerol of step (b).

Relatedly the invention provides a method of increasing triacylglycerolcontent in a transformed host cell comprising:

-   -   (a) providing a transformed host cell comprising:        -   (i) at least one gene encoding a diacylglycerol            acyltransferase-1 enzyme comprising all of the amino acid            motifs as set forth in:            -   1) SEQ ID NO:31;            -   2) SEQ ID NO:32;            -   3) SEQ ID NO:33;            -   4) SEQ ID NO:34;            -   5) SEQ ID NO:35;            -   6) SEQ ID NO:36; and            -   7) SEQ ID NO:37;        -   under the control of suitable regulatory sequences; and,        -   (ii) a source of fatty acids;    -   (b) growing the cell of step (a) under conditions whereby the at        least one gene encoding a diacylglycerol acyltransferase 1        enzyme is expressed, resulting in the transfer of the fatty        acids to triacylglycerol; and    -   (c) optionally recovering the triacylglycerol of step (b).

In similar fashion the invention provides a method of increasingtriacylglycerol content in a transformed host cell comprising:

-   -   (a) providing a transformed host cell comprising:        -   (i) at least one gene encoding a diacylglycerol            acyltransferase 1 enzyme comprising all of the amino acid            motifs as set forth in:            -   1) SEQ ID NO:23;            -   2) SEQ ID NO:24;            -   3) SEQ ID NO:25;            -   4) SEQ ID NO:26;            -   5) SEQ ID NO:27;            -   6) SEQ ID NO:28;            -   7) SEQ ID NO:29; and            -   8) SEQ ID NO:30;        -   under the control of suitable regulatory sequences; and,        -   (ii) a source of fatty acids;    -   (b) growing the cell of step (a) under conditions whereby the at        least one gene encoding a diacylglycerol acyltransferase-1        enzyme is expressed, resulting in the transfer of the fatty        acids to triacylglycerol; and    -   (c) optionally recovering the triacylglycerol of step (b).

Alternatively the invention provides a method of increasing the ω-3 orω-6 fatty acid content of triacylglycerols in a transformed host cellcomprising:

-   -   (a) providing a transformed host cell comprising:        -   (i) genes encoding a functional ω-3%-6 fatty acid            biosynthetic pathway; and        -   (ii) at least one gene encoding a diacylglycerol            acyltransferase 1 enzyme comprising all of the amino acid            motifs as set forth in:            -   1) SEQ ID NO:31;            -   2) SEQ ID NO:32;            -   3) SEQ ID NO:33;            -   4) SEQ ID NO:34;            -   5) SEQ ID NO:35;            -   6) SEQ ID NO:36; and            -   7) SEQ ID NO:37;        -   under the control of suitable regulatory sequences;    -   (b) growing the cell of step (a) under conditions whereby the        genes of (i) and (ii) are expressed, resulting in the production        of at least one ω-3 or ω-6 fatty acid and its transfer to        triacylglycerol; and    -   (c) optionally recovering the triacylglycerol of step (b).

In another embodiment the invention provides a method of increasing theω-3 or ω-6 fatty acid content of triacylglycerols in a transformed hostcell comprising:

-   -   (a) providing a transformed host cell comprising:        -   (i) genes encoding a functional ω-3/ω-6 fatty acid            biosynthetic pathway; and        -   (ii) at least one gene encoding a diacylglycerol            acyltransferase 1 enzyme comprising all of the amino acid            motifs as set forth in:            -   1) SEQ ID NO:23;            -   2) SEQ ID NO:24;            -   3) SEQ ID NO:25;            -   4) SEQ ID NO:26;            -   5) SEQ ID NO:27;            -   6) SEQ ID NO:28;            -   7) SEQ ID NO:29; and            -   8) SEQ ID NO:30;        -   under the control of suitable regulatory sequences;    -   (b) growing the cell of step (a) under conditions whereby the        genes of (i) and (ii) are expressed, resulting in the production        of at least one ω-3 or ω-6 fatty acid and its transfer to        triacylglycerol; and    -   (c) optionally recovering the triacylglycerol of step (b).

In one embodiment the invention provides a method for the identificationof a polypeptide having diacylglycerol acyltransferase-1 activitycomprising:

-   -   a) obtaining the amino acid sequence of a polypeptide suspected        of having diacylglycerol acyltransferase-1 activity; and,    -   b) identifying, in the amino acid sequence of the polypeptide of        step (a), the presence of all of the amino acid motif sequences        as set forth in:        -   1) SEQ ID NO:31;        -   2) SEQ ID NO:32;        -   3) SEQ ID NO:33;        -   4) SEQ ID NO:34;        -   5) SEQ ID NO:35;        -   6) SEQ ID NO:36; and        -   7) SEQ ID NO:37;            wherein the presence of all of the motif sequences of            step (a) in the polypeptide is indicative of diacylglycerol            acyltransferase-1 activity.

In another embodiment the invention provides a method for theidentification of a fungal polypeptide having diacylglycerolacyltransferase-1 activity comprising:

-   -   a) obtaining the amino acid sequence of a fungal polypeptide        suspected of having diacylglycerol acyltransferase-1 activity;        and,    -   b) identifying, in the amino acid sequence of the polypeptide of        step (a), the presence of all of the amino acid motif sequences        as set forth in:        -   1) SEQ ID NO:23;        -   2) SEQ ID NO:24;        -   3) SEQ ID NO:25;        -   4) SEQ ID NO:26;        -   5) SEQ ID NO:27;        -   6) SEQ ID NO:28;        -   7) SEQ ID NO:29; and        -   8) SEQ ID NO:30;            wherein the presence of all of the motif sequences of            step (a) in the polypeptide is indicative of diacylglycerol            acyltransferase-1 activity.

BRIEF DESCRIPTION OF THE DRAWINGS AND SEQUENCE DESCRIPTIONS

FIG. 1 shows a schematic illustration of the biochemical mechanism forlipid accumulation in oleaginous yeast.

FIG. 2 illustrates the ω-3 and ω-6 fatty acid biosynthetic pathways.

FIG. 3 illustrates the construction of plasmid vector pY5 for geneexpression in Yarrowia lipolytica.

FIG. 4 provides plasmid maps for the following plasmids: (A) pY20; (B)pLV13; (C) pMDGAT1-17; and (D) pZUF-Mod-1.

FIG. 5 diagrams the development of various Yarrowia lipolytica strainsproducing up to 15% EPA in the total lipid fraction.

FIG. 6 provides plasmid maps for the following: (A) pKUNF12T6E; (B)pDMW232; (C) pZP3L37; (D) pY37/F15; and (E) pKO2UM26E.

FIG. 7 provides plasmid maps for the following: (A) PZKUM; (B)pKO2UF2PE; (C) pZKUT16; (D) pZP217+Ura; and (E) pZUF17.

FIGS. 8 a, 8 b, 8 c, 8 d, 8 e, 8 f, 8 g and 8 h are an alignment ofDGAT1 proteins using the Megalign program of DNASTAR using Clustal W.

The invention can be more fully understood from the following detaileddescription and the accompanying sequence descriptions, which form apart of this application.

The following sequences comply with 37 C.F.R. §1.821-1.825(“Requirements for Patent Applications Containing Nucleotide Sequencesand/or Amino Acid Sequence Disclosures—the Sequence Rules”) and areconsistent with World Intellectual Property Organization (WIPO) StandardST.25 (1998) and the sequence listing requirements of the EPO and PCT(Rules 5.2 and 49.5(a-bis), and Section 208 and Annex C of theAdministrative Instructions). The symbols and format used for nucleotideand amino acid sequence data comply with the rules set forth in 37C.F.R. §1.822.

SEQ ID NOs:1-38, 112-115, 117-120, 122, 123, 125, 126, 130, 131,133-136, 140, 141 and 169-174 are ORFs encoding genes or proteins (orportions thereof) or protein motifs, as identified in Table 1. TABLE 1Summary of Gene and Protein SEQ ID Numbers Nucleic acid ProteinDescription and Abbreviation SEQ ID NO. SEQ ID NO. Yarrowia lipolyticaDGAT2  1 (2119 bp)  2 (514 AA) (“Yl DGAT2”)  3 (1380 bp)  4 (459 AA)  5(1068 bp)  6 (355 AA) Yarrowia lipolytica PDAT  7 (2326 bp)  8 (648 AA)(“Yl PDAT”) Yarrowia lipolytica YALI-CDS2011.1  9 (1632 bp)  10 (543 AA)(see also GenBank Accession No. NC_006072, bases 974607-976238,locus_tag = “YALI0F06578g”) Yarrowia lipolytica YALI-CDS2141.1  11 (1581bp)  12 (526 AA) (see also GenBank Accession No. CR382130, bases1026155-1027735, locus_tag = “YALI0D07986g”) Yarrowia lipolytica DGAT1 13 (1578 bp)  14 (526 AA) (“Yl DGAT1”) Yarrowia lipolytica ARE2 (“YlARE2”)  15 (1632 bp)  16 (543 AA) Mortierella alpina DGAT1  17 (1578 bp) 18 (525 AA) (“Ma DGAT1”) Mortierella alpina DGAT1 - internal 175 (604bp) — cDNA fragment Neurospora crassa DGAT1 —  19 (533 AA) (“Nc DAGAT1”)Gibberella zeae DGAT1 —  20 (499 AA) (“Fm DAGAT1”) Magnaporthe griseaDGAT1 —  21 (503 AA) (“Mg DAGAT1”) Aspergillus nidulans DGAT1 —  22 (458AA) (“An DAGAT1”) Fungal DGAT1 motif #1 — 23 Fungal DGAT1 motif #2 — 24Fungal DGAT1 motif #3 — 25 Fungal DGAT1 motif #4 — 26 Fungal DGAT1 motif#5 — 27 Fungal DGAT1 motif #6 — 28 Fungal DGAT1 motif #7 — 29 FungalDGAT1 motif #8 — 30 Universal DGAT1 motif #1 — 31 Universal DGAT1 motif#3 — 32 Universal DGAT1 motif #4 — 33 Universal DGAT1 motif #5 — 34Universal DGAT1 motif #6 — 35 Universal DGAT1 motif #7 — 36 UniversalDGAT1 motif #8 — 37 Fungal DGAT2 motif — 38 Synthetic elongase genederived from 112 (957 bp) 113 (318 AA) Mortierella alpina,codon-optimized for expression in Yarrowia lipolytica Synthetic Δ6desaturase, derived from 114 (1374 bp) 115 (457 AA) Mortierella alpina,codon-optimized for expression in Yarrowia lipolytica Fusariummoniliforme Δ12 desaturase 117 (1434 bp) 118 (477 AA) Synthetic elongasegene derived from 119 (819 bp) 120 (272 AA) Thraustochytrium aureum,codon- optimized for expression in Yarrowia lipolytica Mortierellaalpina Δ5 desaturase 122 (1341 bp) 123 (446 AA) Synthetic Δ17 desaturasegene derived 125 (1077 bp) 126 (358 AA) from Saprolegnia diclina, codon-optimized for expression in Yarrowia lipolytica Yarrowia lipolytica Δ12desaturase 130 (1936 bp) 131 (419 AA) Mortieralla isabellina Δ12desaturase 133 (1203 bp) 134 (400 AA) Mortierella alpina Δ6 desaturase“B” 135 (1521 bp) 136 (458 AA) Synthetic C₁₆ elongase gene derived 140(804 bp) 141 (267 AA) from Rattus norvegicus, codon- optimized forexpression in Yarrowia lipolytica Mus musculus DGAT2 — 169 (388 AA) (“MmDGAT1”) Glycine max DGAT1 (“Gm DGAT1”) — 170 (504 AA) Ara bidopsisthaliana DGAT1 — 171 (520 AA) (“At DGAT1”) Oryza sativa DGAT1 (“OsDGAT1”) — 172 (500 AA) Perilla frutescens DGAT1 — 173 (534 AA) (“PfDGAT1”) Triticum aestivum DGAT1 — 174 (508 AA) (Ta DGAT1”)

SEQ ID NOs:55, 82, 110, 121, 124, 128, 129, 137-139, 144, 164, 165 and168 are plasmids as identified in Table 2. TABLE 2 Summary of PlasmidSEQ ID Numbers Corresponding Plasmid FIG. SEQ ID NO pY20 4A  55 (8,196bp) pLV13 4B  82 (5,105 bp) pKUNF12T6E 6A 110 (12,649 bp) pDMW232 6B 121(10,945 bp) pZP3L37 6C 124 (12,690 bp) pY37/F15 6D 128 (8,194 bp)pKO2UM26E 6E 129 (10,448 bp) pZKUM 7A 137 (4,313 bp) pKO2UF2PE 7B 138(10,838 bp) pZKUT16 7C 139 (5,833 bp) pZP2I7 + Ura 7D 144 (7,822 bp)pZUF17 7E 164 (8,165 bp) pMDGAT1-17 4C 165 (8,666 bp) pZUF-MOD-1 4D 168(7,323 bp)

SEQ ID NOs:39 and 40 correspond to primers TEF5′ and TEF3′,respectively, used to isolate the TEF promoter.

SEQ ID NOs:41 and 42 correspond to primers XPR5′ and XPR3′,respectively, used to isolate the XPR2 transcriptional terminator.

SEQ ID NOs:43-54 correspond to primers YL5, YL6, YL9, YL10, YL7, YL8,YL3, YL4, YL1, YL2, YL61 and YL62, respectively, used for plasmidconstruction.

SEQ ID NO:56 corresponds to a 1 kB DNA fragment (amino acid sequenceprovided as SEQ ID NO:57) containing the E. coli hygromycin resistancegene.

SEQ ID NO:58 corresponds to a 1.7 kB DNA fragment containing theYarrowia Ura3 gene (amino acid sequence provided as SEQ ID NO:59), whichwas amplified with primers KU5 and KU3 (SEQ ID NOs:60 and 61,respectively).

SEQ ID NOs:62 and 64 are the degenerate primers identified as P7 and P8,respectively, used for the isolation of a Yarrowia lipolytica DGAT2.

SEQ ID NOs:63 and 65 are the amino acid consensus sequences thatcorrespond to the degenerate primers P7 and P8, respectively.

SEQ ID NOs:66-68 correspond to primers P80, P81 and LinkAmp Primer1,respectively, used for chromosome walking.

SEQ ID NOs:69-72 correspond to primers P95, P96, P97 and P98,respectively, used for targeted disruption of the Y. lipolytica DGAT2gene.

SEQ ID NOs:73-75 correspond to primers P115, P116 and P112,respectively, used to screen for targeted integration of the disruptedY. lipolytica DGAT2 gene.

SEQ ID NOs:76 and 78 are the degenerate primers identified as P26 andP27, respectively, used for the isolation of the Y. lipolytica PDAT.

SEQ ID NOs:77 and 79 are the amino acid consensus sequences thatcorrespond to degenerate primers P26 and P27, respectively.

SEQ ID NOs:80, 81, 83 and 84 correspond to primers P39, P42, P41 andP40, respectively, used for targeted disruption of the Y. lipolyticaPDAT gene.

SEQ ID NOs:85-88 correspond to primers P51, P52, P37 and P38,respectively, used to screen for targeted integration of the disruptedY. lipolytica PDAT gene.

SEQ ID NO:89 corresponds to primer P79, used to amplify the full-lengthY. lipolytica DGAT2 gene from rescued plasmids.

SEQ ID NOs:90 and 91 correspond to primers P84 and P85, respectively,used to amplify the full-length Y. lipolytica PDAT gene from rescuedplasmids.

SEQ ID NOs:92 and 93 are the degenerate primers identified as P201 andP203, respectively, used for the isolation of the Y. lipolytica DGAT1.

SEQ ID NOs:94-99 correspond to primers P214, P215, P216, P217, P218 andP219, respectively, used for targeted disruption of the Y. lipolyticaDGAT1 gene.

SEQ ID NOs:100 and 101 correspond to primers P226 and P227,respectively, used to screen for targeted integration of the disruptedY. lipolytica DGAT1 gene.

SEQ ID NOs:102 and 103 are the degenerate primers identified as P205 andP208, respectively, used for isolation of the Y. lipolytica ARE2.

SEQ ID NOs:104-109 correspond to primers P220, P221, P222, P223, P224and P225, respectively, used for targeted disruption of the Y.lipolytica ARE2 gene.

SEQ ID NOs:111, 116, 127 and 132 correspond to the following Yarrowialipolytica promoters, respectively: fructose-bisphosphatealdolase+intron (FBAIN; 973 bp), fructose-bisphosphate aldolase (FBA;1001 bp), fructose-bisphosphate aldolase+modified intron (FBAINm; 924bp), glycerol-3-phosphate acyltransferase (GPAT; 1130 bp).

SEQ ID NOs:142 and 143 correspond to primers P239 and P240,respectively, used for sequencing of the Y. lipolytica DGAT1 ORF.

SEQ ID NOs:145-147 correspond to BD-Clontech Creator Smarte cDNA librarykit primers SMART IV oligonucleotide, CDSIII/3′ PCR primer and 5′-PCRprimer.

SEQ ID NO:148 corresponds to the M13 forward primer used for sequencingof the M. alpina cDNA library.

SEQ ID NO:149 corresponds to the partial cDNA sequence (601 bp) encodingthe putative M. alpina DGAT1 gene.

SEQ ID NOs:150 and 151 correspond to primers MARE2-N1 and MARE2-N²,respectively, used for cloning the 5′-end region of the putative M.alpina DGAT1 gene.

SEQ ID NOs:152 and 153 correspond to the Genome Walker adaptor fromClonTech's Universal GenomeWalker™ Kit, used for genome-walking.

SEQ ID NOs:154 and 155 correspond to primers AP1 and AP2, respectively,used for genome-walking to isolate the 5′-end region of the M. alpinaDGAT1.

SEQ ID NO:156 corresponds to the 5′-end sequence (1683 bp) of the M.alpina DGAT1 cDNA fragment.

SEQ ID NOs:157 and 158 correspond to primers ARE-N3-1 and ARE-N-3-2,respectively, used for cloning the 3′-end region of the putative M.alpina DGAT1 gene.

SEQ ID NOs:159 and 160 correspond to primers AP and UAP, respectively,used for genome-walking to isolate the 3′-end region of the M. alpinaDGAT1.

SEQ ID NO:161 corresponds to the 3′-end sequence (184 bp) of the M.alpina DGAT1 cDNA fragment.

SEQ ID NOs:162 and 163 correspond to primers MACAT-F1 and MACAT-R,respectively, used for cloning of the M. alpina DGAT1 ORF.

SEQ ID NOs:166 and 167 correspond to primers pzuf-mod1 and pzuf-mod2,respectively, used for creating “control” plasmid pZUF-MOD-1.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the subject invention, Applicants have isolated andconfirmed the identity of a Yarrowia lipolytica gene encoding adiacylglycerol acyltransferase (DGAT1) enzyme useful for transferringfatty acids into storage triacylglycerols (TAGs). Orthologous genes havealso been isolated from Mortierella alpina and identified in Neurosporacrassa, Gibberella zeae PH-1, Magnaporthe grisea and Aspergillusnidulans. Furthermore, the present invention provides motifs for readilyidentifying other DGAT1 genes from fungal organisms. In anotherembodiment, the Yarrowia lipolytica gene encoding anacyl-CoA:sterol-acyltransferase (ARE2) enzyme has been isolated. Each ofthese genes may be useful to alter the quantity of long chainpolyunsaturated fatty acids (PUFAs) produced in transformant oleaginousyeasts.

The importance of PUFAs are undisputed. For example, certain PUFAs areimportant biological components of healthy cells and are recognized as:“essential” fatty acids that cannot be synthesized de novo in mammalsand instead must be obtained either in the diet or derived by furtherdesaturation and elongation of linoleic acid (LA) or α-linolenic acid(ALA); constituents of plasma membranes of cells, where they may befound in such forms as phospholipids or TAGs; necessary for properdevelopment (particularly in the developing infant brain) and for tissueformation and repair; and, precursors to several biologically activeeicosanoids of importance in mammals (e.g., prostacyclins, eicosanoids,leukotrienes, prostaglandins). Additionally, a high intake of long-chainω-3 PUFAs produces cardiovascular protective effects (Dyerberg, J. etal., Amer. J. Clin Nutr. 28:958-966 (1975); Dyerberg, J. et al., Lancet2(8081):117-119 (Jul. 15, 1978); Shimokawa, H., World Rev Nutr Diet,88:100-108 (2001); von Schacky, C. and Dyerberg, J., World Rev NutrDiet, 88:90-99 (2001)). And, numerous other studies documentwide-ranging health benefits conferred by administration of ω-3 and/orω-6 fatty acids against a variety of symptoms and diseases (e.g.,asthma, psoriasis, eczema, diabetes, cancer).

As such, the subject invention finds many applications. PUFAs, orderivatives thereof, accumulated by the methodology disclosed herein canbe used as dietary substitutes, or supplements, particularly infantformulas, for patients undergoing intravenous feeding or for preventingor treating malnutrition. Alternatively, the purified PUFAs (orderivatives thereof) may be incorporated into cooking oils, fats ormargarines formulated so that in normal use the recipient would receivethe desired amount for dietary supplementation. The PUFAs may also beincorporated into infant formulas, nutritional supplements or other foodproducts and may find use as anti-inflammatory or cholesterol loweringagents. Optionally, the compositions may be used for pharmaceutical use(human or veterinary). In this case, the PUFAs are generallyadministered orally but can be administered by any route by which theymay be successfully absorbed, e.g., parenterally (e.g., subcutaneously,intramuscularly or intravenously), rectally, vaginally or topically(e.g., as a skin ointment or lotion).

Supplementation of humans or animals with PUFAs produced by recombinantmeans can result in increased levels of the added PUFAs, as well astheir metabolic progeny. For example, treatment with ARA can result notonly in increased levels of ARA, but also downstream products of ARAsuch as prostaglandins. Complex regulatory mechanisms can make itdesirable to combine various PUFAs, or add different conjugates ofPUFAs, in order to prevent, control or overcome such mechanisms toachieve the desired levels of specific PUFAs in an individual.

DEFINITIONS

In this disclosure, a number of terms and abbreviations are used. Thefollowing definitions are provided.

“Open reading frame” is abbreviated ORF.

“Polymerase chain reaction” is abbreviated PCR.

“American Type Culture Collection” is abbreviated ATCC.

“Polyunsaturated fatty acid(s)” is abbreviated PUFA(s).

“Acyl-CoA:sterol-acyltransferase” is abbreviated ARE2.

“Phospholipid:diacylglycerol acyltransferase” is abbreviated PDAT.

“Diacylglycerol acyltransferase” is abbreviated DAG AT or DGAT.

“Diacylglycerol” is abbreviated DAG.

“Triacylglycerols” are abbreviated TAGs.

“Co-enzyme A” is abbreviated CoA.

The term “fatty acids” refers to long chain aliphatic acids (alkanoicacids) of varying chain length, from about C₁₂ to C₂₂ (although bothlonger and shorter chain-length acids are known). The predominant chainlengths are between C₁₆ and C₂₂. The structure of a fatty acid isrepresented by a simple notation system of “X:Y”, where X is the totalnumber of carbon (C) atoms in the particular fatty acid and Y is thenumber of double bonds.

Generally, fatty acids are classified as saturated or unsaturated. Theterm “saturated fatty acids” refers to those fatty acids that have no“double bonds” between their carbon backbone. In contrast, “unsaturatedfatty acids” have “double bonds” along their carbon backbones (which aremost commonly in the cis-configuration). “Monounsaturated fatty acids”have only one “double bond” along the carbon backbone (e.g., usuallybetween the 9^(th) and 10^(th) carbon atom as for palmitoleic acid(16:1) and oleic acid (18:1)), while “polyunsaturated fatty acids” (or“PUFAs”) have at least two double bonds along the carbon backbone (e.g.,between the 9^(th) and 10^(th), and 12^(th) and 13^(th) carbon atoms forlinoleic acid (18:2); and between the 9^(th) and 10^(th), 12^(th) and13^(th), and 15^(th) and 16^(th) for α-linolenic acid (18:3)).

“PUFAs” can be classified into two major families (depending on theposition (n) of the first double bond nearest the methyl end of thefatty acid carbon chain). Thus, the “omega-6 fatty acids” (ω-6 or n-6)have the first unsaturated double bond six carbon atoms from the omega(methyl) end of the molecule and additionally have a total of two ormore double bonds, with each subsequent unsaturation occurring 3additional carbon atoms toward the carboxyl end of the molecule. Incontrast, the “omega-3 fatty acids” (ω-3 or n-3) have the firstunsaturated double bond three carbon atoms away from the omega end ofthe molecule and additionally have a total of three or more doublebonds, with each subsequent unsaturation occurring 3 additional carbonatoms toward the carboxyl end of the molecule.

For the purposes of the present disclosure, the omega-reference systemwill be used to indicate the number of carbons, the number of doublebonds and the position of the double bond closest to the omega carbon,counting from the omega carbon (which is numbered 1 for this purpose).This nomenclature is shown below in Table 3, in the column titled“Shorthand Notation”. The remainder of the Table summarizes the commonnames of ω-3 and ω-6 fatty acids, the abbreviations that will be usedthroughout the specification and each compounds' chemical name. TABLE 3Nomenclature Of Polyunsaturated Fatty Acids Shorthand Common NameAbbreviation Chemical Name Notation Linoleic LA cis-9,12-octadecadienoic18:2 ω-6 γ-Linoleic GLA cis-6,9,12- 18:3 ω-6 octadecatrienoicEicosadienoic EDA cis-11,14-eicosadienoic 20:2 ω-6 Dihomo-γ- DGLAcis-8,11,14- 20:3 ω-6 Linoleic eicosatrienoic Arachidonic ARAcis-5,8,11,14- 20:4 ω-6 eicosatetraenoic α-Linolenic ALA cis-9,12,15-18:3 ω-3 octadecatrienoic Stearidonic STA cis-6,9,12,15- 18:4 ω-3octadecatetraenoic Eicosatrienoic ETrA cis-11,14,17- 20:3 ω-3eicosatrienoic Eicosatetraenoic ETA cis-8,11,14,17- 20:4 ω-3eicosatetraenoic Eicosapentaenoic EPA cis-5,8,11,14,17- 20:5 ω-3eicosapentaenoic Docosapentaenoic DPA cis-7,10,13,16,19- 22:5 ω-3docosapentaenoic Docosahexaenoic DHA cis-4,7,10,13,16,19- 22:6 ω-3docosahexaenoic

“Microbial oils” or “single cell oils” are those oils naturally producedby microorganisms (e.g., algae, oleaginous yeasts and filamentous fungi)during their lifespan. The term “oil” refers to a lipid substance thatis liquid at 25° C. and usually polyunsaturated. In contrast, the term“fat” refers to a lipid substance that is solid at 25° C. and usuallysaturated.

“Lipid bodies” refer to lipid droplets that usually are bounded byspecific proteins and a monolayer of phospholipid. These organelles aresites where most organisms transport/store neutral lipids. Lipid bodiesare thought to arise from microdomains of the endoplasmic reticulum thatcontain TAG-biosynthesis enzymes; and, their synthesis and size appearto be controlled by specific protein components.

“Neutral lipids” refer to those lipids commonly found in cells in lipidbodies as storage fats and oils and are so called because at cellularpH, the lipids bear no charged groups. Generally, they are completelynon-polar with no affinity for water. Neutral lipids generally refer tomono-, di-, and/or triesters of glycerol with fatty acids, also calledmonoacylglycerol, diacylglycerol or TAG, respectively (or collectively,acylglycerols). A hydrolysis reaction must occur to release free fattyacids from acylglycerols.

The terms “triacylglycerol”, “oil” and “TAGs” refer to neutral lipidscomposed of three fatty acyl residues esterified to a glycerol molecule(and such terms will be used interchangeably throughout the presentdisclosure herein). Such oils can contain long chain PUFAs, as well asshorter saturated and unsaturated fatty acids and longer chain saturatedfatty acids. Thus, “oil biosynthesis” generically refers to thesynthesis of TAGs in the cell.

The term “DAG AT” refers to a diacylglycerol acyltransferase (also knownas an acyl-CoA-diacylglycerol acyltransferase or a diacylglycerolO-acyltransferase) (EC 2.3.1.20). This enzyme is responsible for theconversion of acyl-CoA and 1,2-diacylglycerol to TAG and CoA (therebyinvolved in the terminal step of TAG biosynthesis). Two families of DAGAT enzymes exist: DGAT1 and DGAT2. The former family shares homologywith the acyl-CoA:cholesterol acyltransferase (ACAT) gene family, whilethe latter family is unrelated (Lardizabal et al., J. Biol. Chem.276(42):38862-28869 (2001)).

The term “PDAT” refers to a phospholipid:diacylglycerol acyltransferaseenzyme (EC 2.3.1.158). This enzyme is responsible for the transfer of anacyl group from the sn-2 position of a phospholipid to the sn-3 positionof 1,2-diacylglycerol, thus resulting in lysophospholipid and TAG(thereby involved in the terminal step of TAG biosynthesis). This enzymediffers from DGAT (EC 2.3.1.20) by synthesizing TAG via anacyl-CoA-independent mechanism.

The term “ARE2” refers to an acyl-CoA:sterol-acyltransferase enzyme (EC2.3.1.26; also known as a sterol-ester synthase 2 enzyme), catalyzingthe following reaction: acyl-CoA+cholesterol=CoA+cholesterol ester.

The term “PUFA biosynthetic pathway enzyme” refers to any of thefollowing enzymes (and genes which encode said enzymes) associated withthe biosynthesis of a PUFA, including: a Δ4 desaturase, a Δ5 desaturase,a Δ6 desaturase, a Δ12 desaturase, a Δ15 desaturase, a Δ17 desaturase, aΔ9 desaturase, a Δ8 desaturase and/or an elongase(s).

The term “ω-3/ω-6 fatty acid biosynthetic pathway” refers to a set ofgenes which, when expressed under the appropriate conditions encodeenzymes that catalyze the production of either or both ω-3 and ω-6 fattyacids. Typically the genes involved in the ω-3/ω-6 fatty acidbiosynthetic pathway encode some or all of the following enzymes: Δ12desaturase, Δ6 desaturase, elongase(s), Δ5 desaturase, Δ17 desaturase,Δ15 desaturase, Δ9 desaturase, Δ8 desaturase and Δ4 desaturase. Arepresentative pathway is illustrated in FIG. 2, providing for theconversion of oleic acid through various intermediates to DHA, whichdemonstrates how both ω-3 and ω-6 fatty acids may be produced from acommon source. The pathway is naturally divided into two portions whereone portion will generate ω-3 fatty acids and the other portion, onlyω-6 fatty acids. That portion that only generates ω-3 fatty acids willbe referred to herein as the ω-3 fatty acid biosynthetic pathway,whereas that portion that generates only ω-6 fatty acids will bereferred to herein as the ω-6 fatty acid biosynthetic pathway.

The term “functional” as used herein in context with the ω-3/ω-6 fattyacid biosynthetic pathway means that some (or all of) the genes in thepathway express active enzymes. It should be understood that “ω-3/ω-6fatty acid biosynthetic pathway” or “functional ω-3/ω-6 fatty acidbiosynthetic pathway” does not imply that all the genes listed in theabove paragraph are required, as a number of fatty acid products willonly require the expression of a subset of the genes of this pathway.

The term “desaturase” refers to a polypeptide that can desaturate, i.e.,introduce a double bond, in one or more fatty acids to produce a mono-or polyunsaturated fatty acid. Despite use of the omega-reference systemthroughout the specification in reference to specific fatty acids, it ismore convenient to indicate the activity of a desaturase by countingfrom the carboxyl end of the substrate using the delta-system. Ofparticular interest herein are: Δ12 desaturases that desaturate a fattyacid between the 12^(th) and 13^(th) carbon atoms numbered from thecarboxyl-terminal end of the molecule and that catalyze the conversionof oleic acid to LA; Δ15 desaturases that catalyze the conversion of LAto ALA; Δ17 desaturases that catalyze the conversion of ARA to EPAand/or DGLA to ETA; Δ6 desaturases that catalyze the conversion of LA toGLA and/or ALA to STA; Δ5 desaturases that catalyze the conversion ofDGLA to ARA and/or ETA to EPA; Δ4 desaturases that catalyze theconversion of DPA to DHA; Δ8 desaturases that catalyze the conversion ofEDA to DGLA and/or ETrA to ETA; and Δ9 desaturases that catalyze theconversion of palmitate to palmitoleic acid (16:1) and/or stearate tooleic acid (18:1).

The term “elongase” refers to a polypeptide that can elongate a fattyacid carbon chain to produce an acid that is 2 carbons longer than thefatty acid substrate that the elongase acts upon. This process ofelongation occurs in a multi-step mechanism in association with fattyacid synthase, whereby CoA is the acyl carrier (Lassner et al., ThePlant Cell 8:281-292 (1996)). Briefly, malonyl-CoA is condensed with along-chain acyl-CoA to yield CO₂ and a β-ketoacyl-CoA (where the acylmoiety has been elongated by two carbon atoms). Subsequent reactionsinclude reduction to β-hydroxyacyl-CoA, dehydration to an enoyl-CoA anda second reduction to yield the elongated acyl-CoA. Examples ofreactions catalyzed by elongases are the conversion of GLA to DGLA, STAto ETA, and EPA to DPA. Accordingly, elongases can have differentspecificities. For example, a C_(16/18) elongase will prefer a C₁₆substrate, a C_(18/20) elongase will prefer a C₁₈ substrate and aC_(20/22) elongase will prefer a C₂₀ substrate. In like manner, a Δ9elongase is able to catalyze the conversion of LA and ALA to EDA andETrA, respectively.

The term “oleaginous” refers to those organisms that tend to store theirenergy source in the form of lipid (Weete, In: Fungal LipidBiochemistry, 2^(nd) Ed., Plenum, 1980). Generally, the cellular oilcontent of these microorganisms follows a sigmoid curve, wherein theconcentration of lipid increases until it reaches a maximum at the latelogarithmic or early stationary growth phase and then graduallydecreases during the late stationary and death phases (Yongmanitchai andWard, Appl. Environ. Microbiol. 57:419-25 (1991)).

The term “oleaginous yeast” refers to those microorganisms classified asyeasts that can make oil. Generally, the cellular oil or triacylglycerolcontent of oleaginous microorganisms follows a sigmoid curve, whereinthe concentration of lipid increases until it reaches a maximum at thelate logarithmic or early stationary growth phase and then graduallydecreases during the late stationary and death phases (Yongmanitchai andWard, Appl. Environ. Microbiol. 57:419-25 (1991)). It is not uncommonfor oleaginous microorganisms to accumulate in excess of about 25% oftheir dry cell weight as oil. Examples of oleaginous yeast include, butare no means limited to, the following genera: Yarrowia, Candida,Rhodotorula, Rhodosporidium, Cryptococcus, Trichosporon and Lipomyces.

The term “fermentable carbon source” means a carbon source that amicroorganism will metabolize to derive energy. Typical carbon sourcesof the invention include, but are not limited to: monosaccharides,oligosaccharides, polysaccharides, alkanes, fatty acids, esters of fattyacids, monoglycerides, carbon dioxide, methanol, formaldehyde, formateand carbon-containing amines.

The terms an “isolated nucleic acid fragment” an “isolated nucleic acidmolecule” will be used interchangeably and will mean a polymer of RNA orDNA that is single- or double-stranded, optionally containing synthetic,non-natural or altered nucleotide bases. An isolated nucleic acidfragment in the form of a polymer of DNA may be comprised of one or moresegments of cDNA, genomic DNA or synthetic DNA.

A nucleic acid molecule is “hybridizable” to another nucleic acidmolecule, such as a cDNA, genomic DNA, or RNA molecule, when asingle-stranded form of the nucleic acid molecule can anneal to theother nucleic acid molecule under the appropriate conditions oftemperature and solution ionic strength. Hybridization and washingconditions are well known and exemplified in Sambrook, J., Fritsch, E.F. and Maniatis, T. Molecular Cloning: A Laboratory Manual, 2^(nd) ed.,Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989),particularly Chapter 11 and Table 11.1 therein (entirely incorporatedherein by reference). The conditions of temperature and ionic strengthdetermine the “stringency” of the hybridization. Stringency conditionscan be adjusted to screen for moderately similar fragments (such ashomologous sequences from distantly related organisms), to highlysimilar fragments (such as genes that duplicate functional enzymes fromclosely related organisms). Post-hybridization washes determinestringency conditions. One set of preferred conditions uses a series ofwashes starting with 6×SSC, 0.5% SDS at room temperature for 15 min,then repeated with 2×SSC, 0.5% SDS at 45° C. for 30 min, and thenrepeated twice with 0.2×SSC, 0.5% SDS at 50° C. for 30 min. A morepreferred set of stringent conditions uses higher temperatures in whichthe washes are identical to those above except for the temperature ofthe final two 30 min washes in 0.2×SSC, 0.5% SDS was increased to 60° C.Another preferred set of highly stringent conditions uses two finalwashes in 0.1×SSC, 0.1% SDS at 65° C. An additional set of stringentconditions include hybridization at 0.1×SSC, 0.1% SDS, 65° C. and washedwith 2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1% SDS, for example.

Hybridization requires that the two nucleic acids contain complementarysequences, although depending on the stringency of the hybridization,mismatches between bases are possible. The appropriate stringency forhybridizing nucleic acids depends on the length of the nucleic acids andthe degree of complementation, variables well known in the art. Thegreater the degree of similarity or homology between two nucleotidesequences, the greater the value of Tm for hybrids of nucleic acidshaving those sequences. The relative stability (corresponding to higherTm) of nucleic acid hybridizations decreases in the following order:RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotidesin length, equations for calculating Tm have been derived (see Sambrooket al., supra, 9.50-9.51). For hybridizations with shorter nucleicacids, i.e., oligonucleotides, the position of mismatches becomes moreimportant, and the length of the oligonucleotide determines itsspecificity (see Sambrook et al., supra, 11.7-11.8). In one embodimentthe length for a hybridizable nucleic acid is at least about 10nucleotides. Preferably a minimum length for a hybridizable nucleic acidis at least about 15 nucleotides; more preferably at least about 20nucleotides; and most preferably the length is at least about 30nucleotides. Furthermore, the skilled artisan will recognize that thetemperature and wash solution salt concentration may be adjusted asnecessary according to factors such as length of the probe.

A “substantial portion” of an amino acid or nucleotide sequence is thatportion comprising enough of the amino acid sequence of a polypeptide orthe nucleotide sequence of a gene to putatively identify thatpolypeptide or gene, either by manual evaluation of the sequence by oneskilled in the art, or by computer-automated sequence comparison andidentification using algorithms such as BLAST (Basic Local AlignmentSearch Tool; Altschul, S. F., et al., J. Mol. Biol. 215:403-410 (1993)).In general, a sequence of ten or more contiguous amino acids or thirtyor more nucleotides is necessary in order to putatively identify apolypeptide or nucleic acid sequence as homologous to a known protein orgene. Moreover, with respect to nucleotide sequences, gene specificoligonucleotide probes comprising 20-30 contiguous nucleotides may beused in sequence-dependent methods of gene identification (e.g.,Southern hybridization) and isolation (e.g., in situ hybridization ofbacterial colonies or bacteriophage plaques). In addition, shortoligonucleotides of 12-15 bases may be used as amplification primers inPCR in order to obtain a particular nucleic acid fragment comprising theprimers. Accordingly, a “substantial portion” of a nucleotide sequencecomprises enough of the sequence to specifically identify and/or isolatea nucleic acid fragment comprising the sequence. The instantspecification teaches partial or complete amino acid and nucleotidesequences encoding one or more particular yeast and fungal proteins. Theskilled artisan, having the benefit of the sequences as reported herein,may now use all or a substantial portion of the disclosed sequences forpurposes known to those skilled in this art. Accordingly, the instantinvention comprises the complete sequences as reported in theaccompanying Sequence Listing, as well as substantial portions of thosesequences as defined above.

The term “complementary” is used to describe the relationship betweennucleotide bases that are capable of hybridizing to one another. Forexample, with respect to DNA, adenosine is complementary to thymine andcytosine is complementary to guanine. Accordingly, the instant inventionalso includes isolated nucleic acid fragments that are complementary tothe complete sequences as reported in the accompanying Sequence Listing,as well as those substantially similar nucleic acid sequences.

The term “percent identity”, as known in the art, is a relationshipbetween two or more polypeptide sequences or two or more polynucleotidesequences, as determined by comparing the sequences. In the art,“identity” also means the degree of sequence relatedness betweenpolypeptide or polynucleotide sequences, as the case may be, asdetermined by the match between strings of such sequences. “Identity”and “similarity” can be readily calculated by known methods, includingbut not limited to those described in: 1.) Computational MolecularBiology (Lesk, A. M., Ed.) Oxford University: NY (1988); 2.)Biocomputing: Informatics and Genome Projects (Smith, D. W., Ed.)Academic: NY (1993); 3.) Computer Analysis of Sequence Data, Part I(Griffin, A. M., and Griffin, H. G., Eds.) Humania: NJ (1994); 4.)Sequence Analysis in Molecular Biology (von Heinje, G., Ed.) Academic(1987); and 5.) Sequence Analysis Primer (Gribskov, M. and Devereux, J.,Eds.) Stockton: NY (1991). Preferred methods to determine identity aredesigned to give the best match between the sequences tested. Methods todetermine identity and similarity are codified in publicly availablecomputer programs. Sequence alignments and percent identity calculationsmay be performed using the Megalign program of the LASERGENEbioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiplealignment of the sequences is performed using the Clustal method ofalignment (Higgins and Sharp, CABIOS. 5:151-153 (1989)) with defaultparameters (GAP PENALTY=10, GAP LENGTH PENALTY=10), unless otherwisespecified. Default parameters for pairwise alignments using the Clustalmethod are: KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5.

Suitable nucleic acid fragments (isolated polynucleotides of the presentinvention) encode polypeptides that are at least about 70% identical,preferably at least about 75% identical, and more preferably at leastabout 80% identical to the amino acid sequences reported herein.Preferred nucleic acid fragments encode amino acid sequences that areabout 85% identical to the amino acid sequences reported herein. Morepreferred nucleic acid fragments encode amino acid sequences that are atleast about 90% identical to the amino acid sequences reported herein.Most preferred are nucleic acid fragments that encode amino acidsequences that are at least about 95% identical to the amino acidsequences reported herein. Suitable nucleic acid fragments not only havethe above homologies but typically encode a polypeptide having at least50 amino acids, preferably at least 100 amino acids, more preferably atleast 150 amino acids, still more preferably at least 200 amino acids,and most preferably at least 250 amino acids.

“Codon degeneracy” refers to the nature in the genetic code permittingvariation of the nucleotide sequence without affecting the amino acidsequence of an encoded polypeptide. The skilled artisan is well aware ofthe “codon-bias” exhibited by a specific host cell in usage ofnucleotide codons to specify a given amino acid. Therefore, whensynthesizing a gene for improved expression in a host cell, it isdesirable to design the gene such that its frequency of codon usageapproaches the frequency of preferred codon usage of the host cell.

The term “codon-optimized”, as it refers to genes or coding regions ofnucleic acid molecules, refers to modification of codons such that thealtered codons reflect the typical codon usage of the host organismwithout altering the polypeptide for which the DNA codes.

“Chemically synthesized”, as related to a sequence of DNA, means thatthe component nucleotides were assembled in vitro. Manual chemicalsynthesis of DNA may be accomplished using well-established procedures;or automated chemical synthesis can be performed using one of a numberof commercially available machines. “Synthetic genes” can be assembledfrom oligonucleotide building blocks that are chemically synthesizedusing procedures known to those skilled in the art. These buildingblocks are ligated and annealed to form gene segments that are thenenzymatically assembled to construct the entire gene. Accordingly, thegenes can be tailored for optimal gene expression based on optimizationof nucleotide sequence to reflect the codon bias of the host cell. Theskilled artisan appreciates the likelihood of successful gene expressionif codon usage is biased towards those codons favored by the host.Determination of preferred codons can be based on a survey of genesderived from the host cell, where sequence information is available.

“Gene” refers to a nucleic acid fragment that expresses a specificprotein, including regulatory sequences preceding (5′ non-codingsequences) and following (3′ non-coding sequences) the coding sequence.“Native gene” refers to a gene as found in nature with its ownregulatory sequences. “Chimeric gene” refers to any gene that is not anative gene, comprising regulatory and coding sequences that are notfound together in nature. Accordingly, a chimeric gene may compriseregulatory sequences and coding sequences that are derived fromdifferent sources, or regulatory sequences and coding sequences derivedfrom the same source, but arranged in a manner different than that foundin nature. “Endogenous gene” refers to a native gene in its naturallocation in the genome of an organism. A “foreign” gene refers to a genenot normally found in the host organism, but that is introduced into thehost organism by gene transfer. Foreign genes can comprise native genesinserted into a non-native organism, or chimeric genes. A “transgene” isa gene that has been introduced into the genome by a transformationprocedure. A “codon-optimized gene” is a gene having its frequency ofcodon usage designed to mimic the frequency of preferred codon usage ofthe host cell.

“Coding sequence” refers to a DNA sequence that codes for a specificamino acid sequence. “Suitable regulatory sequences” refer to nucleotidesequences located upstream (5′ non-coding sequences), within, ordownstream (3′ non-coding sequences) of a coding sequence, and whichinfluence the transcription, RNA processing or stability, or translationof the associated coding sequence. Regulatory sequences may includepromoters, translation leader sequences, introns, polyadenylationrecognition sequences, RNA processing sites, effector binding sites andstem-loop structures.

“Promoter” refers to a DNA sequence capable of controlling theexpression of a coding sequence or functional RNA. In general, a codingsequence is located 3′ to a promoter sequence. Promoters may be derivedin their entirety from a native gene, or be composed of differentelements derived from different promoters found in nature, or evencomprise synthetic DNA segments. It is understood by those skilled inthe art that different promoters may direct the expression of a gene indifferent tissues or cell types, or at different stages of development,or in response to different environmental or physiological conditions.Promoters that cause a gene to be expressed in most cell types at mosttimes are commonly referred to as “constitutive promoters”. It isfurther recognized that since in most cases the exact boundaries ofregulatory sequences have not been completely defined, DNA fragments ofdifferent lengths may have identical promoter activity.

The term “3′ non-coding sequences” or “transcription terminator” refersto DNA sequences located downstream of a coding sequence. This includespolyadenylation recognition sequences and other sequences encodingregulatory signals capable of affecting mRNA processing or geneexpression. The polyadenylation signal is usually characterized byaffecting the addition of polyadenylic acid tracts to the 3′ end of themRNA precursor. The 3′ region can influence the transcription, RNAprocessing or stability, or translation of the associated codingsequence.

“RNA transcript” refers to the product resulting from RNApolymerase-catalyzed transcription of a DNA sequence. When the RNAtranscript is a perfect complementary copy of the DNA sequence, it isreferred to as the primary transcript or it may be a RNA sequencederived from post-transcriptional processing of the primary transcriptand is referred to as the mature RNA. “Messenger RNA” or “mRNA” refersto the RNA that is without introns and that can be translated intoprotein by the cell. “cDNA” refers to a double-stranded DNA that iscomplementary to, and derived from, mRNA. “Sense” RNA refers to RNAtranscript that includes the mRNA and so can be translated into proteinby the cell. “Antisense RNA” refers to a RNA transcript that iscomplementary to all or part of a target primary transcript or mRNA andthat blocks the expression of a target gene (U.S. Pat. No. 5,107,065; WO99/28508). The complementarity of an antisense RNA may be with any partof the specific gene transcript, i.e., at the 5′ non-coding sequence, 3′non-coding sequence, or the coding sequence. “Functional RNA” refers toantisense RNA, ribozyme RNA, or other RNA that is not translated and yethas an effect on cellular processes.

The term “operably linked” refers to the association of nucleic acidsequences on a single nucleic acid fragment so that the function of oneis affected by the other. For example, a promoter is operably linkedwith a coding sequence when it is capable of affecting the expression ofthat coding sequence (i.e., the coding sequence is under thetranscriptional control of the promoter). Coding sequences can beoperably linked to regulatory sequences in sense or antisenseorientation.

The term “expression”, as used herein, refers to the transcription andstable accumulation of sense (mRNA) or antisense RNA derived from thenucleic acid fragment(s) of the invention. Expression may also refer totranslation of mRNA into a polypeptide.

“Transformation” refers to the transfer of a nucleic acid molecule intoa host organism, resulting in genetically stable inheritance. Thenucleic acid molecule may be a plasmid that replicates autonomously, forexample; or, it may integrate into the genome of the host organism. Hostorganisms containing the transformed nucleic acid fragments are referredto as “transgenic” or “recombinant” or “transformed” organisms.

The terms “plasmid”, “vector” and “cassette” refer to an extrachromosomal element often carrying genes that are not part of thecentral metabolism of the cell, and usually in the form of circulardouble-stranded DNA fragments. Such elements may be autonomouslyreplicating sequences, genome integrating sequences, phage or nucleotidesequences, linear or circular, of a single- or double-stranded DNA orRNA, derived from any source, in which a number of nucleotide sequenceshave been joined or recombined into a unique construction which iscapable of introducing a promoter fragment and DNA sequence for aselected gene product along with appropriate 3′ untranslated sequenceinto a cell. “Transformation cassette” refers to a specific vectorcontaining a foreign gene(s) and having elements in addition to theforeign gene(s) that facilitate transformation of a particular hostcell. “Expression cassette” refers to a specific vector containing aforeign gene(s) and having elements in addition to the foreign gene(s)that allow for enhanced expression of that gene in a foreign host.

The term “homologous recombination” refers to the exchange of DNAfragments between two DNA molecules (during cross over). The fragmentswhich are exchanged are flanked by sites of identical nucleotidesequences between the two DNA molecules (i.e., “regions of homology”).The term “regions of homology” refer to stretches of nucleotide sequenceon nucleic acid fragments that participate in homologous recombinationthat have homology to each other. Effective homologous recombinationwill generally take place where these regions of homology are at leastabout 10 bp in length where at least about 50 bp in length is preferred.Typically fragments that are intended for recombination contain at leasttwo regions of homology where targeted gene disruption or replacement isdesired.

The term “sequence analysis software” refers to any computer algorithmor software program that is useful for the analysis of nucleotide oramino acid sequences. “Sequence analysis software” may be commerciallyavailable or independently developed. Typical sequence analysis softwarewill include, but is not limited to: 1.) the GCG suite of programs(Wisconsin Package Version 9.0, Genetics Computer Group (GCG), Madison,Wis.); 2.) BLASTP, BLASTN, BLASTX (Altschul et al., J. Mol. Biol.215:403-410 (1990)); 3.) DNASTAR (DNASTAR, Inc. Madison, Wis.); 4.)Sequencher (Gene Codes Corporation, Ann Arbor, Mich.); and 5.) the FASTAprogram incorporating the Smith-Waterman algorithm (W. R. Pearson,Comput. Methods Genome Res., [Proc. Int. Symp.] (1994), Meeting Date1992, 111-20. Suhai, Sandor, Ed. Plenum: New York, N.Y.). Within thecontext of this application it will be understood that where sequenceanalysis software is used for analysis, that the results of the analysiswill be based on the “default values” of the program referenced, unlessotherwise specified. As used herein “default values” will mean any setof values or parameters that originally load with the software whenfirst initialized.

The term “conserved domain” or “motif” means a set of amino acidsconserved at specific positions along an aligned sequence ofevolutionarily related proteins. While amino acids at other positionscan vary between homologous proteins, amino acids that are highlyconserved at specific positions indicate amino acids that are essentialin the structure, the stability, or the activity of a protein. Becausethey are identified by their high degree of conservation in alignedsequences of a family of protein homologues, they can be used asidentifiers, or “signatures”, to determine if a protein with a newlydetermined sequence belongs to a previously identified protein family.Motifs that are universally found in DGAT1 enzymes (i.e., animal, plantsand fungi) are provided as SEQ ID NOs:31-37; motifs found in DGAT1 sthat are specific to fungal organisms are provided as SEQ ID NOs:23-30.

Standard recombinant DNA and molecular cloning techniques used hereinare well known in the art and are described by Sambrook, J., Fritsch, E.F. and Maniatis, T., Molecular Cloning: A Laboratory Manual, 2^(nd) ed.,Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989)(hereinafter “Maniatis”); by Silhavy, T. J., Bennan, M. L. and Enquist,L. W., Experiments with Gene Fusions, Cold Spring Harbor Laboratory:Cold Spring Harbor, N.Y. (1984); and by Ausubel, F. M. et al., CurrentProtocols in Molecular Biology, published by Greene Publishing Assoc.and Wiley-Interscience (1987).

Microbial Biosynthesis of Fatty Acids and Triacylglycerols

In general, lipid accumulation in oleaginous microorganisms is triggeredin response to the overall carbon to nitrogen ratio present in thegrowth medium (FIG. 1). When cells have exhausted available nitrogensupplies (e.g., when the carbon to nitrogen ratio is greater than about40), the depletion of cellular adenosine monophosphate (AMP) leads tothe cessation of AMP-dependent isocitrate dehydrogenase activity in themitochondria and the accumulation of citrate, transport of citrate intothe cytosol, and subsequent cleavage of the citrate by ATP-citrate lyaseto yield acetyl-CoA and oxaloacetate. Acetyl-CoA is the principlebuilding block for de novo biosynthesis of fatty acids. Although anycompound that can effectively be metabolized to acetyl-CoA can serve asa precursor of fatty acids, glucose is the primary source of carbon inthis type of reaction (FIG. 1). Glucose is converted to pyruvate viaglycolysis, and pyruvate is then transported into the mitochondria whereit can be converted to acetyl-CoA by pyruvate dehydrogenase (“PD”).Since acetyl-CoA can not be transported directly across themitochondrial membrane into the cytoplasm, the two carbons fromacetyl-CoA condense with oxaloacetate to yield citrate (catalyzed bycitrate synthase). Citrate is transported directly into the cytoplasm,where it is cleaved by ATP-citrate lyase to regenerate acetyl-CoA andoxaloacetate. The oxaloacetate reenters the tricarboxylic acid cycle,via conversion to malate.

The synthesis of malonyl-CoA is the first committed step of fatty acidbiosynthesis, which takes place in the cytoplasm. Malonyl-CoA isproduced via carboxylation of acetyl-CoA by acetyl-CoA carboxylase(“ACC”). Fatty acid synthesis is catalyzed by a multi-enzyme fatty acidsynthase complex (“FAS”) and occurs by the condensation of eighttwo-carbon fragments (acetyl groups from acetyl-CoA) to form a 16-carbonsaturated fatty acid, palmitate. More specifically, FAS catalyzes aseries of 7 reactions, which involve the following (Smith, S. FASEB J,8(15):1248-59 (1994)):

-   -   1. Acetyl-CoA and malonyl-CoA are transferred to the acyl        carrier peptide (ACP) of FAS. The acetyl group is then        transferred to the malonyl group, forming β-ketobutyryl-ACP and        releasing CO₂.    -   2. The β-ketobutyryl-ACP undergoes reduction (via β-ketoacyl        reductase) and dehydration (via β-hydroxyacyl dehydratase) to        form a trans-monounsaturated fatty acyl group.    -   3. The double bond is reduced by NADPH, yielding a saturated        fatty-acyl group two carbons longer than the initial one. The        butyryl-group's ability to condense with a new malonyl group and        repeat the elongation process is then regenerated.    -   4. When the fatty acyl group becomes 16 carbons long, a        thioesterase activity hydrolyses it, releasing free palmitate        (16:0).

Whereas palmitate synthesis occurs in the cytosol, formation of longerchain saturated and unsaturated fatty acid derivates occur in both themitochondria and endoplasmic reticulum (ER), wherein the ER is thedominant system. Specifically, palmitate (16:0) is the precursor ofstearic (18:0), palmitoleic (16:1) and oleic (18:1) acids through theaction of elongases and desaturases. For example, palmitate and stearateare converted to their unsaturated derivatives, palmitoleic (16:1) andoleic (18:1) acids, respectively, by the action of a Δ9 desaturase.

TAGs (the primary storage unit for fatty acids) are formed by a seriesof reactions that involve: 1.) the esterification of one molecule ofacyl-CoA to glycerol-3-phosphate via an acyltransferase to producelysophosphatidic acid; 2.) the esterification of a second molecule ofacyl-CoA via an acyltransferase to yield 1,2-diacylglycerol phosphate(commonly identified as phosphatidic acid); 3.) removal of a phosphateby phosphatidic acid phosphatase to yield 1,2-diacylglycerol (DAG); and4.) the addition of a third fatty acid by the action of anotheracyltransferase (e.g., PDAT, DGAT1 or DGAT2) to form TAG (FIG. 1).

A wide spectrum of fatty acids can be incorporated into TAGs, includingsaturated and unsaturated fatty acids and short-chain and long-chainfatty acids. Some non-limiting examples of fatty acids that can beincorporated into TAGs by acyltransferases (e.g., DAG ATs or PDAT)include:capric (10:0), lauric (12:0), myristic (14:0), palmitic (16:0),palmitoleic (16:1), stearic (18:0), oleic (18:1), vaccenic (18:1),linoleic (18:2), eleostearic (18:3), γ-linolenic (18:3), α-linolenic(18:3), stearidonic (18:4), arachidic (20:0), eicosadienoic (20:2),dihomo-γ-linoleic (20:3), eicosatrienoic (20:3), arachidonic (20:4),eicosa-tetraenoic (20:4), eicosa-pentaenoic (20:5), behenic (22:0),docosa-pentaenoic (22:5), docosa-hexaenoic (22:6), lignoceric (24:0),nervonic (24:1), cerotic (26:0) and montanic (28:0) fatty acids. Inpreferred embodiments of the present invention, incorporation of PUFAsinto TAG is most desirable.

Acyltransferases and their Role in the Terminal Step of Tag Biosynthesis

Genes Encoding DGAT1

Historically, DGAT1 (responsible for the third acyl transferasereaction, wherein an acyl-CoA group is transferred from acyl-CoA to thesn-3 position of DAG to form TAG) was thought to be the only enzymespecifically involved in TAG synthesis. This enzyme was known to behomologous to acyl-CoA:cholesterol acyltransferases (ACATs); however,recent studies have identified a new family of DAG acyltransferase (DAGAT) enzymes that are unrelated to the ACAT gene family. Thus,nomenclature now distinguishes between the DAG AT enzymes that arerelated to the ACAT gene family (DGAT1 family) versus those that areunrelated (DGAT2 family) (Lardizabal et al., J. Biol. Chem.276(42):38862-28869 (2001)).

Many genes encoding DGAT1 enzymes have been identified through geneticmeans and the DNA sequences of some of these genes are publiclyavailable. For example, some non-limiting examples include the followingGenBank Accession Numbers: AY445635 (olive); AF384160 (mouse);NM_(—)053437 (Norway rat); NM_(—)174693 (cow); AY116586 (pig); AY327327and AY327326 (Toxoplasma gondii); AF298815 (Perilla frutescens); andAF164434 (Brassica napus). Additionally, the patent literature providesmany additional DNA sequences of DGAT1 genes (and/or details concerningseveral of the genes above and their methods of isolation). See, forexample: U.S. Pat. No. 6,100,077 (human); U.S. Pat. No. 6,552,250(Brassica); U.S. Pat. No. 6,344,548 (human, mouse, Arabidopsis); US2004/0088759A1 (plant); and US 2004/0078836A1 (Farese et al.).

Genes Encoding DGAT2

Members of the DGAT2 family appear to be present in all major phyla ofeukaryotes (fungi, plants, animals and basal eukaryotes). As such, manygenes encoding DGAT2 enzymes have been identified through genetic meansand the DNA sequences of some of these genes are publicly available. Forexample, some non-limiting examples include the following GenBankAccession Numbers: NC_(—)001147 (locus NP_(—)014888; Saccharomycescerevisiae); NM_(—)012079 (human); NM_(—)127503, AF051849 and AJ238008(Arabidopsis thaliana); NM_(—)026384, NM_(—)010046 and AB057816 (mouse);AY093657 (pig); AB062762 (rat); AF221132 (Caenorhabditis elegans);AF391089 and AF391090 (Mortierella ramanniana); AF129003 (Nicotianatabacum); and, AF251794 and AF164434 (Brassica napus). Additionally, thepatent literature provides many additional DNA sequences of DGAT2 genes(and/or details concerning several of the genes above and their methodsof isolation). See, for example: US 2003/124126 (Cases et al.); WO2001/034814 (Banas et al.); and US 2003/115632, US 2003/0028923 and US2004/0107459 (Lardizabal et al.). The work of Lardizabal et al. includesDNA sequences of DGAT2s from, e.g., Mortierella ramanniana, Neurosporacrassa, Saccharomyces cerevisiae, Hordeum vulgare, Zea mays, Glycinemax, Triticum aestivum, Drosophilia, Homo sapiens, Schizosaccharomycespombe, Candida albicans and Arabidopsis thaliana.

Most recently, a DGAT2 enzyme from the oleaginous yeast Yarrowialipolytica has been isolated and characterized in co-pending U.S. patentapplication Ser. No. 10/882,760 (incorporated entirely herein byreference). Briefly, following cloning of a partial putative DGAT2 DNAfragment from Y. lipolytica, targeted disruption of the endogenous Y.lipolytica gene was carried out to test the identity of the fragment.Lower oil content in the disrupted strain confirmed that the nativeDGAT2 activity was eliminated. Subsequently, a full-length Y. lipolyticaDGAT2 gene (2119 bp; SEQ ID NO:1) was assembled, which included threenested open reading frames: 1.) ORF 1: nucleotides +291 to +1835 of SEQID NO:1, corresponding to the protein encoded by SEQ ID NO:2 (514 aminoacid residues); 2.) ORF 2: nucleotides +456 to +1835 of SEQ ID NO:1,corresponding to SEQ ID NO:3 (1380 bases) and the protein encoded by SEQID NO:4 (459 amino acid residues); and 3.) ORF 3: nucleotides +768 to+1835 of SEQ ID NO:1, corresponding to SEQ ID NO:5 (1068 bases) and theprotein encoded by SEQ ID NO:6 (355 amino acid residues).

Genes Encoding PDAT

TAG synthesis can also occur in the absence of acyl-CoA, via theacyl-CoA-independent PDAT enzyme, as recently discovered by Dahlqvist etal. (Proc. Nat. Acad. Sci. (USA) 97:6487-6492 (2000)) and Oelkers et al.(J. Biol. Chem. 275:15609-15612 (2000)). Specifically, PDAT removes anacyl group from the sn-2 position of a phosphotidylcholine substrate fortransfer to the sn-3 position of DAG to produce TAG; and, although thefunction of PDAT is not as well characterized as DGAT2, PDAT has beenpostulated to play a major role in removing “unusual” fatty acids fromphospholipids in some oilseed plants (Banas, A. et al., Biochem. Soc.Trans. 28(6):703-705 (2000)).

PDAT is structurally related to the lecithin:cholesterol acyltransferase(LCAT) family of proteins. Several genes encoding PDAT enzymes have beenidentified through genetic means and the DNA sequences of some of thesegenes are publicly available. For example, some non-limiting examplesinclude the following GenBank Accession Numbers: P40345 (Saccharomycescerevisiae); O94680 and NP_(—)596330 (Schizosaccharomyces pombe); and,NP_(—)190069 and AB006704 [gi:2351069] (Arabidopsis thaliana).Additionally, the patent literature provides many additional DNAsequences of PDAT genes (and/or details concerning several of the genesabove and their methods of isolation); see, for example, WO 2000/060095(Dahlqvist et al.).

and, particularly relevant to the disclosure herein, a PDAT enzyme fromthe oleaginous yeast Yarrowia lipolytica has been isolated andcharacterized in co-pending U.S. patent application Ser. No. 10/882,760in a manner similar to that described above for DGAT2. Again, lower oilcontent in a strain having a disruption in the putative PDAT geneconfirmed that the native PDAT activity was eliminated. Subsequently, afull-length Y. lipolytica PDAT gene (2326 bp; SEQ ID NO:7) wasassembled.

Genes Encoding ARE2

The process of sterol esterification in yeast was first studied by H.Yang et al. (Science. 272(5266):1353-1356 (1996)), wherein it wasdiscovered that two genes (ARE1 and ARE2) encode ACAT-related enzymesthat permit the esterification of cholesterol. The DNA sequences of onlya few ARE2 genes are publicly available. For example, see GenBankAccession Numbers: □876L2 (Saccharomyces bayanus), P53629 (S.cerevisiae) and Q10269 (Schizosaccharomyces pombe).

Interaction between PDAT, DGAT1, DGAT2 and ARE2

In S. cerevisiae, four genes (i.e., ARE1, ARE2, DGA1 [encoding DGAT2]and LRO1 [encoding PDAT]), contribute to oil biosynthesis. PDAT andDGAT2 are responsible for up to approximately 95% of oil biosynthesis(Sandager, L. et al., J. Biol. Chem. 277(8):6478-6482 (2002); Oelkerset. al. J. Biol. Chem. 277:8877 (2002)).

Surprisingly, according to the work described in co-pending U.S. patentapplication Ser. No. 10/882,760 in Yarrowia lipolytica, PDAT and DGAT2appeared to only be partially responsible for oil biosynthesis. Thus, itwas apparent that at least one other DAG AT must play a role in TAGformation. As described in the Application herein, oil biosynthesis inthe yeast Yarrowia lipolytica requires the activity of PDAT, DGAT1 andDGAT2, while ARE2 may additionally be a minor contributor to oilbiosynthesis. This is in marked contrast to the enzymes responsible foroil biosynthesis in S. cerevisiae, where only DGAT2 and PDAT are themajor DAG ATs.

Biosynthesis of Omega-3 and Omega-6 Polyunsaturated Fatty Acids

The metabolic process that converts LA to GLA, DGLA and ARA (the ω-6pathway) and ALA to STA, ETA, EPA, DPA and DHA (the ω-3 pathway)involves elongation of the carbon chain through the addition oftwo-carbon units and desaturation of the molecule through the additionof double bonds (FIG. 2). This requires a series of desaturation andelongation enzymes. Specifically, oleic acid is converted to LA (18:2),the first of the ω-6 fatty acids, by the action of a Δ12 desaturase.Subsequent ω-6 fatty acids are produced as follows: 1.) LA is convertedto GLA by the action of a Δ6 desaturase; 2.) GLA is converted to DGLA bythe action of an elongase; and 3.) DGLA is converted to ARA by theaction of a Δ5 desaturase. In like manner, linoleic acid (LA) isconverted to ALA, the first of the ω-3 fatty acids, by the action of aΔ15 desaturase. Subsequent ω-3 fatty acids are produced in a series ofsteps similar to that for the ω-6 fatty acids. Specifically, 1.) ALA isconverted to STA by the activity of a Δ6 desaturase; 2.) STA isconverted to ETA by the activity of an elongase; and 3.) ETA isconverted to EPA by the activity of a Δ5 desaturase. Alternatively, ETAand EPA can be produced from DGLA and ARA, respectively, by the activityof a Δ17 desaturase. EPA can be further converted to DHA by the activityof an elongase and a Δ4 desaturase.

In alternate embodiments, a Δ9 elongase is able to catalyze theconversion of LA and ALA to EDA and ETrA, respectively. A Δ8 desaturasethen converts these products to DGLA and ETA, respectively.

Many microorganisms, including algae, bacteria, molds, fungi and yeasts,can synthesize PUFAs and omega fatty acids in the ordinary course ofcellular metabolism. Particularly well-studied are fungi includingSchizochytrium aggregatm, species of the genus Thraustochytrium andMortierella alpina. Additionally, many dinoflagellates (Dinophyceaae)naturally produce high concentrations of PUFAs. As such, a variety ofdesaturase and elongase genes involved in PUFA production have beenidentified through genetic means and the DNA sequences of some of thesegenes are publicly available (non-limiting examples are as follows):AY131238, Y055118, AY055117, AF296076, AF007561, L11421, NM_(—)031344,AF465283, AF465281, AF110510, AF465282, AF419296, AB052086, AJ250735,AF126799, AF126798 (A6 desaturases); AF199596, AF226273, AF320509,AB072976, AF489588, AJ510244, AF419297, AF07879, AF067654, AB022097 (Δ5desaturases); AF489589.1, AY332747 (Δ4 fatty acid desaturases);AAG36933, AF110509, AB020033, AAL13300, AF417244, AF161219, AY332747,AAG36933, AF110509, AB020033, AAL13300, AF417244, AF161219, X86736,AF240777, AB007640, AB075526, AP002063 (Δ12 desaturases); NP_(—)441622,BAA18302, BAA02924, AAL36934 (Δ15 desaturases); AF338466, AF438199,E11368, E11367, D83185, U90417, AF085500, AY504633, NM_(—)069854,AF230693 (Δ9 desaturases); and AX464731, NM_(—)119617, NM_(—)134255,NM_(—)134383, NM_(—)134382, NM_(—)068396, NM_(—)068392, NM_(—)070713,NM_(—)068746, NM_(—)064685 (elongases).

Additionally, the patent literature provides many additional DNAsequences of genes (and/or details concerning several of the genes aboveand their methods of isolation) involved in PUFA production. See, forexample: U.S. Pat. No. 5,968,809 (Δ6 desaturases); U.S. Pat. No.5,972,664 and U.S. Pat. No. 6,075,183 (Δ5 desaturases); WO 91/13972 andU.S. Pat. No. 5,057,419 (Δ9 desaturases); WO 93/11245 (Δ15 desaturases);WO 94/11516, U.S. Pat. No. 5,443,974 and WO 03/099216 (A12 desaturases);WO 00/12720 and U.S. 2002/0139974A1 (elongases); U.S. 2003/0196217 A1(Δ17 desaturase); WO 00/34439 (Δ8 desaturases); and, WO 02/090493 (Δ4desaturases). Each of these patents and applications are hereinincorporated by reference in their entirety.

Depending upon the host cell, the availability of substrate, and thedesired end product(s), several desaturases and elongases are ofinterest for use in production of PUFAs. Considerations for choosing aspecific polypeptide having desaturase or elongase activity include: 1.)the substrate specificity of the polypeptide; 2.) whether thepolypeptide or a component thereof is a rate-limiting enzyme; 3.)whether the desaturase or elongase is essential for synthesis of adesired PUFA; and/or 4.) co-factors required by the polypeptide. Theexpressed polypeptide preferably has parameters compatible with thebiochemical environment of its location in the host cell. For example,the polypeptide may have to compete for substrate with other enzymes inthe host cell. Analyses of the K_(M) and specific activity of thepolypeptide are therefore considered in determining the suitability of agiven polypeptide for modifying PUFA production in a given host cell.The polypeptide used in a particular host cell is one that can functionunder the biochemical conditions present in the intended host cell butotherwise can be any polypeptide having desaturase or elongase activitycapable of modifying the desired fatty acid substrate.

Sequence Identification of DGAT1 Acyltransferases

Despite the availability of several genes encoding DGAT1 (supra) thatcould be used for heterologous expression in oleaginous yeast (e.g.,Yarrowia lipolytica), expression of a native enzyme is sometimespreferred over a heterologous (or “foreign”) enzyme. This preference mayoccur because: 1.) the native enzyme is optimized for interaction withother enzymes and proteins in the cell; and 2.) heterologous genes areunlikely to share the same codon preference in the host organism.Knowledge of the sequences of a host organism's native DGAT1 alsofacilitates disruption of the homologous chromosomal genes by targeteddisruption. And, as the present invention has shown, understanding ofthe complete complement of acyltransferase genes that enable TAGsynthesis in an organism enables one to readily manipulate the oilcontent that the host organism produces in a variety of ways.

Comparison of the Yarrowia lipolytica DGAT1 nucleotide base (SEQ IDNO:13) and deduced amino acid (SEQ ID NO:14) sequences to publicdatabases reveals that the most similar known sequences are about 55%identical to the amino acid sequence of DGAT1 reported herein over alength of 526 amino acids using the BLASTP method of alignment with alow complexity filter and the following parameters: Expect value=10,matrix=Blosum 62 (Altschul, et al., Nucleic Acids Res.25:3389-3402(1997))).

Comparison of the Mortierella alpina DGAT1 nucleotide base (SEQ IDNO:17) and deduced amino acid (SEQ ID NO:18) sequences to publicdatabases reveals that the most similar known sequences are about 49%identical to the amino acid sequence of DGAT1 reported herein over alength of 525 amino acids using the BLASTP method of alignment(Altschul, et al., supra). Furthermore, comparison of the Y. lipolyticaDGAT1 to the M. alpina DGAT1 revealed the two proteins to be 32.4%identical.

Similarly, comparison of the Neurospora crassa DGAT1 amino acid (SEQ IDNO:19) sequence to the Yarrowia lipolytica DGAT1 amino acid (SEQ IDNO:14) reveals about 37% identity over a length of 533 amino acids;comparison of the Gibberella zeae PH-1 DGAT1 amino acid (SEQ ID NO:20)sequence to the Y. lipolytica DGAT1 amino acid (SEQ ID NO:14) revealsabout 38.1% identity over a length of 499 amino acids; comparison of theMagnaporthe grisea DGAT1 amino acid (SEQ ID NO:21) sequence to the Y.lipolytica DGAT1 amino acid (SEQ ID NO:14) reveals about 36.2% identityover a length of 503 amino acids; and, comparison of the Aspergillusnidulans DGAT1 amino acid (SEQ ID NO:22) sequence to the Y. lipolyticaDGAT1 amino acid (SEQ ID NO:14) reveals about 41.7% identity over alength of 458 amino acids.

More preferred DGAT1 amino acid fragments are at least about 70%-80%identical to the sequences herein (i.e., SEQ ID NOs:14, 18, 19, 20, 21,22), where those sequences that are 85%-90% identical are particularlysuitable and those sequences that are about 95% identical are mostpreferred. Similarly, preferred DGAT1 encoding nucleic acid sequencescorresponding to the instant ORFs (i.e., SEQ ID NOs:13 and 17) are thoseencoding active proteins and which are at least about 70%-80% identicalto the nucleic acid sequences encoding DGAT1 reported herein, wherethose sequences that are 85%-90% identical are particularly suitable andthose sequences that are about 95% identical are most preferred.

Sequence Identification of an ARE2 Acyltransferase

Comparison of the Yarrowia lipolytica ARE2 nucleotide base (SEQ IDNO:15) and deduced amino acid (SEQ ID NO:16) sequences to publicdatabases reveals that the most similar known sequences are about 44%identical to the amino acid sequence of ARE2 reported herein over alength of 543 amino acids using the BLASTP method of alignment with alow complexity filter and the following parameters: Expect value=10,matrix=Blosum 62 (Altschul, et al., Nucleic Acids Res.25:3389-3402(1997)). More preferred ARE2 amino acid fragments are atleast about 70%-80% identical to the sequences herein, where thosesequences that are 85%-90% identical are particularly suitable and thosesequences that are about 95% identical are most preferred. Similarly,preferred ARE2 encoding nucleic acid sequences corresponding to theinstant ORFs are those encoding active proteins and which are at leastabout 70%-80% identical to the nucleic acid sequences encoding ARE2reported herein, where those sequences that are 85%-90% identical areparticularly suitable and those sequences that are about 95% identicalare most preferred.

Isolation of Homologs

Each of the acyltransferase nucleic acid fragments of the instantinvention may be used to isolate genes encoding homologous proteins fromthe same or other microbial species. Isolation of homologous genes usingsequence-dependent protocols is well known in the art. Examples ofsequence-dependent protocols include, but are not limited to: 1.)methods of nucleic acid hybridization; 2.) methods of DNA and RNAamplification, as exemplified by various uses of nucleic acidamplification technologies [e.g., polymerase chain reaction (PCR),Mullis et al., U.S. Pat. No. 4,683,202; ligase chain reaction (LCR),Tabor, S. et al., Proc. Acad. Sci. USA 82:1074 (1985); or stranddisplacement amplification (SDA), Walker, et al., Proc. Natl. Acad. Sci.U.S.A., 89:392 (1992)]; and 3.) methods of library construction andscreening by complementation.

For example, genes encoding similar proteins or polypeptides to theDGAT1s and ARE2s described herein could be isolated directly by usingall or a portion of the instant nucleic acid fragments as DNAhybridization probes to screen libraries from any desired yeast orfungus using methodology well known to those skilled in the art.Specific oligonucleotide probes based upon the instant nucleic acidsequences can be designed and synthesized by methods known in the art(Maniatis, supra). Moreover, the entire sequences can be used directlyto synthesize DNA probes by methods known to the skilled artisan (e.g.,random primers DNA labeling, nick translation or end-labelingtechniques), or RNA probes using available in vitro transcriptionsystems. In addition, specific primers can be designed and used toamplify a part of (or full-length of) the instant sequences. Theresulting amplification products can be labeled directly duringamplification reactions or labeled after amplification reactions, andused as probes to isolate full-length DNA fragments under conditions ofappropriate stringency.

Typically, in PCR-type amplification techniques, the primers havedifferent sequences and are not complementary to each other. Dependingon the desired test conditions, the sequences of the primers should bedesigned to provide for both efficient and faithful replication of thetarget nucleic acid. Methods of PCR primer design are common and wellknown in the art (Thein and Wallace, “The use of oligonucleotides asspecific hybridization probes in the Diagnosis of Genetic Disorders”, inHuman Genetic Diseases: A Practical Approach, K. E. Davis Ed., (1986) pp33-50, IRL: Herndon, Va.; and Rychlik, W., In Methods in MolecularBiology, White, B. A. Ed., (1993) Vol. 15, pp 31-39, PCR Protocols:Current Methods and Applications. Humania: Totowa, N.J.).

Generally two short segments of the instant sequences may be used inpolymerase chain reaction protocols to amplify longer nucleic acidfragments encoding homologous genes from DNA or RNA. The polymerasechain reaction may also be performed on a library of cloned nucleic acidfragments wherein the sequence of one primer is derived from the instantnucleic acid fragments, and the sequence of the other primer takesadvantage of the presence of the polyadenylic acid tracts to the 3′ endof the mRNA precursor encoding microbial genes.

Alternatively, the second primer sequence may be based upon sequencesderived from the cloning vector. For example, the skilled artisan canfollow the RACE protocol (Frohman et al., PNAS USA 85:8998 (1988)) togenerate cDNAs by using PCR to amplify copies of the region between asingle point in the transcript and the 3′ or 5′ end. Primers oriented inthe 3′ and 5′ directions can be designed from the instant sequences.Using commercially available 3′ RACE or 5′ RACE systems (e.g., BRL,Gaithersburg, Md.), specific 3′ or 5′ cDNA fragments can be isolated(Ohara et al., PNAS USA 86:5673 (1989); Loh et al., Science 243:217(1989)).

Alternatively, the instant DGAT1 and ARE2 sequences may be employed ashybridization reagents for the identification of homologs. The basiccomponents of a nucleic acid hybridization test include a probe, asample suspected of containing the gene or gene fragment of interest,and a specific hybridization method. Probes of the present invention aretypically single-stranded nucleic acid sequences that are complementaryto the nucleic acid sequences to be detected. Probes are “hybridizable”to the nucleic acid sequence to be detected. The probe length can varyfrom 5 bases to tens of thousands of bases, and will depend upon thespecific test to be done. Typically a probe length of about 15 bases toabout 30 bases is suitable. Only part of the probe molecule need becomplementary to the nucleic acid sequence to be detected. In addition,the complementarity between the probe and the target sequence need notbe perfect. Hybridization does occur between imperfectly complementarymolecules with the result that a certain fraction of the bases in thehybridized region are not paired with the proper complementary base.

Hybridization methods are well defined. Typically the probe and samplemust be mixed under conditions that will permit nucleic acidhybridization. This involves contacting the probe and sample in thepresence of an inorganic or organic salt under the proper concentrationand temperature conditions. The probe and sample nucleic acids must bein contact for a long enough time that any possible hybridizationbetween the probe and sample nucleic acid may occur. The concentrationof probe or target in the mixture will determine the time necessary forhybridization to occur. The higher the probe or target concentration,the shorter the hybridization incubation time needed. Optionally, achaotropic agent may be added. The chaotropic agent stabilizes nucleicacids by inhibiting nuclease activity. Furthermore, the chaotropic agentallows sensitive and stringent hybridization of short oligonucleotideprobes at room temperature (Van Ness and Chen, Nucl. Acids Res.19:5143-5151 (1991)). Suitable chaotropic agents include guanidiniumchloride, guanidinium thiocyanate, sodium thiocyanate, lithiumtetrachloroacetate, sodium perchlorate, rubidium tetrachloroacetate,potassium iodide and cesium trifluoroacetate, among others. Typically,the chaotropic agent will be present at a final concentration of about 3M. If desired, one can add formamide to the hybridization mixture,typically 30-50% (v/v).

Various hybridization solutions can be employed. Typically, thesecomprise from about 20 to 60% volume, preferably 30%, of a polar organicsolvent. A common hybridization solution employs about 30-50% v/vformamide, about 0.15 to 1 M sodium chloride, about 0.05 to 0.1 Mbuffers (e.g., sodium citrate, Tris-HCl, PIPES or HEPES (pH range about6-9)), about 0.05 to 0.2% detergent (e.g., sodium dodecylsulfate), orbetween 0.5-20 mM EDTA, FICOLL (Pharmacia Inc.) (about 300-500 kdal),polyvinylpyrrolidone (about 250-500 kdal) and serum albumin. Alsoincluded in the typical hybridization solution will be unlabeled carriernucleic acids from about 0.1 to 5 mg/mL, fragmented nucleic DNA (e.g.,calf thymus or salmon sperm DNA, or yeast RNA), and optionally fromabout 0.5 to 2% wt/vol glycine. Other additives may also be included,such as volume exclusion agents that include a variety of polarwater-soluble or swellable agents (e.g., polyethylene glycol), anionicpolymers (e.g., polyacrylate or polymethylacrylate) and anionicsaccharidic polymers (e.g., dextran sulfate).

Nucleic acid hybridization is adaptable to a variety of assay formats.One of the most suitable is the sandwich assay format. The sandwichassay is particularly adaptable to hybridization under non-denaturingconditions. A primary component of a sandwich-type assay is a solidsupport. The solid support has adsorbed to it or covalently coupled toit immobilized nucleic acid probe that is unlabeled and complementary toone portion of the sequence.

Availability of the instant nucleotide and deduced amino acid sequencesfacilitates immunological screening of DNA expression libraries.Synthetic peptides representing portions of the instant amino acidsequences may be synthesized. These peptides can be used to immunizeanimals to produce polyclonal or monoclonal antibodies with specificityfor peptides or proteins comprising the amino acid sequences. Theseantibodies can then be used to screen DNA expression libraries toisolate full-length DNA clones of interest (Lerner, R. A. Adv. Immunol.36:1 (1984); Maniatis, supra).

Gene Optimization for Improved Heterologous Expression

It may be desirable to modify the expression of the instantacyltransferases and/or PUFA biosynthetic pathway enzymes to achieveoptimal conversion efficiency of each, according to the specific TAGcomposition that is desired within a specific host organism. As such, avariety of techniques can be utilized to improve and/or optimize theexpression of a polypeptide of interest in an alternative host. Two suchtechniques include codon-optimization and mutagenesis of the gene.

Codon Optimization

As will be appreciated by one skilled in the art, it is frequentlyuseful to modify a portion of the codons encoding a particularpolypeptide that is to be expressed in a foreign host, such that themodified polypeptide uses codons that are preferred by the alternatehost. Use of host-preferred codons can substantially enhance theexpression of the foreign gene encoding the polypeptide.

In general, host-preferred codons can be determined within a particularhost species of interest by examining codon usage in proteins(preferably those expressed in the largest amount) and determining whichcodons are used with highest frequency. Thus, the coding sequence for apolypeptide having acyltransferase activity can be synthesized in wholeor in part using the codons preferred in the host species. All (orportions) of the DNA also can be synthesized to remove any destabilizingsequences or regions of secondary structure that would be present in thetranscribed mRNA. All (or portions) of the DNA also can be synthesizedto alter the base composition to one more preferable in the desired hostcell.

Thus, for example, it may be desirable to modify a portion of the codonsencoding the Mortierella polypeptide having DGAT1 activity, to enhancethe expression of the gene in Yarrowia lipolytica, should the enzyme'ssubstrate specificity be different than that of the Y. lipolytica DGAT1disclosed herein. The codon usage profile and the consensus sequencearound the ‘ATG’ translation initiation codon for this particularorganism (i.e., Y. lipolytica) are taught in co-pending U.S. patentapplication Ser. No. 10/840,478 (herein incorporated entirely byreference); likewise, a method for rapid synthesis of genes optimizedfor expression in Y. lipolytica is also provided.

Mutagenesis

Methods for synthesizing sequences and bringing sequences together arewell established in the literature. For example, in vitro mutagenesisand selection, site-directed mutagenesis, error prone PCR (Melnikov etal., Nucleic Acids Research, 27(4):1056-1062 (Feb. 15, 1999)), “geneshuffling” or other means can be employed to obtain mutations ofnaturally occurring acyltransferase genes. This would permit productionof a polypeptide having acyltransferase activity in vivo with moredesirable physical and kinetic parameters for function in the host cell(e.g., a longer half-life or a higher rate of synthesis of TAGs fromfatty acids).

If desired, the regions of a DGAT1 or ARE2 polypeptide important forenzymatic activity can be determined through routine mutagenesis,expression of the resulting mutant polypeptides and determination oftheir activities. Mutants may include deletions, insertions and pointmutations, or combinations thereof. A typical functional analysis beginswith deletion mutagenesis to determine the N- and C-terminal limits ofthe protein necessary for function, and then internal deletions,insertions or point mutants are made to further determine regionsnecessary for function. Other techniques such as cassette mutagenesis ortotal synthesis also can be used. Deletion mutagenesis is accomplished,for example, by using exonucleases to sequentially remove the 5′ or 3′coding regions. Kits are available for such techniques. After deletion,the coding region is completed by ligating oligonucleotides containingstart or stop codons to the deleted coding region after the 5′ or 3′deletion, respectively. Alternatively, oligonucleotides encoding startor stop codons are inserted into the coding region by a variety ofmethods including site-directed mutagenesis, mutagenic PCR or byligation onto DNA digested at existing restriction sites. Internaldeletions can similarly be made through a variety of methods includingthe use of existing restriction sites in the DNA, by use of mutagenicprimers via site-directed mutagenesis or mutagenic PCR. Insertions aremade through methods such as linker-scanning mutagenesis, site-directedmutagenesis or mutagenic PCR. Point mutations are made throughtechniques such as site-directed mutagenesis or mutagenic PCR.

Chemical mutagenesis also can be used for identifying regions of anacyltransferase polypeptide important for activity. A mutated constructis expressed, and the ability of the resulting altered protein tofunction as an acyltransferase is assayed. Such structure-functionanalysis can determine which regions may be deleted, which regionstolerate insertions, and which point mutations allow the mutant proteinto function in substantially the same way as the native acyltransferase.

All such mutant proteins and nucleotide sequences encoding them that arederived from the DGAT1 and ARE2 genes described herein are within thescope of the present invention.

Microbial Production of Fatty Acids and Triacylglycerols

Microbial production of fatty acids and TAGs has several advantages overpurification from natural sources such as fish or plants. For example:

-   -   1.) Many microbes are known with greatly simplified oil        compositions compared with those of higher organisms, making        purification of desired components easier;    -   2.) Microbial production is not subject to fluctuations caused        by external variables, such as weather and food supply;    -   3.) Microbially produced oil is substantially free of        contamination by environmental pollutants; and,    -   4.) Microbial oil production can be manipulated by controlling        culture conditions, notably by providing particular substrates        for microbially expressed enzymes, or by addition of compounds        or genetic engineering approaches to suppress undesired        biochemical pathways.        With respect to the production of ω-3 and/or ω-6 fatty acids in        particular, and TAGs containing those PUFAs, additional        advantages are incurred since microbes can provide fatty acids        in particular forms that may have specific uses; and,        recombinant microbes provide the ability to alter the naturally        occurring microbial fatty acid profile by providing new        biosynthetic pathways in the host or by suppressing undesired        pathways, thereby increasing levels of desired PUFAs, or        conjugated forms thereof, and decreasing levels of undesired        PUFAs.

Thus, knowledge of the sequences of the present acyltransferase geneswill be useful for manipulating fatty acid biosynthesis and accumulationin oleaginous yeasts, and particularly, in Yarrowia lipolytica. This mayrequire metabolic engineering directly within the fatty acid or TAGbiosynthetic pathways or additional manipulation of pathways thatcontribute carbon to the fatty acid biosynthetic pathway. Methods usefulfor manipulating biochemical pathways are well known to those skilled inthe art.

Metabolic Engineering to Up-Regulate Genes and Biosynthetic PathwaysAffecting Fatty Acid Synthesis and Oil Accumulation in Oleaginous Yeast

It is expected that introduction of chimeric genes encoding theacyltransferases described herein, under the control of the appropriatepromoters, will result in increased transfer of fatty acids to storageTAGs. As such, the present invention encompasses a method for increasingthe TAG content in an oleaginous yeast comprising expressing at leastone acyltransferase enzyme of the present invention in a transformedoleaginous yeast host cell producing a fatty acid, such that the fattyacid is transferred to the TAG pool.

Additional copies of acyltransferase genes (e.g., DGAT1, ARE2) may beintroduced into the host to increase the transfer of fatty acids to theTAG fraction. Expression of the genes also can be increased at thetranscriptional level through the use of a stronger promoter (eitherregulated or constitutive) to cause increased expression, byremoving/deleting destabilizing sequences from either the mRNA or theencoded protein, or by adding stabilizing sequences to the mRNA (U.S.Pat. No. 4,910,141). Yet another approach to increase expression ofheterologous genes is to increase the translational efficiency of theencoded mRNAs by replacement of codons in the native gene with those foroptimal gene expression in the selected host microorganism.

In one specific embodiment, the present invention encompasses a methodof increasing the ω-3 and/or ω-6 fatty acid content of TAGs in anoleaginous yeast, since it is possible to introduce an expressioncassette encoding each of the enzymes necessary for ω-3 and/or ω-6 fattyacid biosynthesis into the organism (since naturally produced PUFAs inthese organisms are limited to 18:2 (i.e., LA), and less commonly 18:3(i.e., ALA) fatty acids). Thus, the method comprises:

-   -   a) providing a transformed oleaginous yeast host cell possessing        genes encoding a functional ω-3%-6 fatty acid biosynthetic        pathway and at least one acyltransferase enzyme of the present        invention;    -   b) growing the yeast cells of step (a) in the presence of a        fermentable carbon substrate, whereby the gene(s) of the ω-3/ω-6        fatty acid biosynthetic pathway and the acyltransferase(s) are        expressed, whereby a ω-3 and/or ω-6 fatty acid is produced, and        whereby the ω-3 and/or ω-6 fatty acid is transferred to TAGs.

A variety of PUFA products can be produced (prior to their transfer toTAGs), depending on the fatty acid substrate and the particular genes ofthe ω-3/ω-6 fatty acid biosynthetic pathway that are transformed intothe host cell. As such, production of the desired fatty acid product canoccur directly (wherein the fatty acid substrate is converted directlyinto the desired fatty acid product without any intermediate steps orpathway intermediates) or indirectly (wherein multiple genes encodingthe PUFA biosynthetic pathway may be used in combination, such that aseries of reactions occur to produce a desired PUFA). Specifically, forexample, it may be desirable to transform an oleaginous yeast with anexpression cassette comprising a Δ12 desaturase, Δ6 desaturase, anelongase, a Δ5 desaturase and a Δ17 desaturase for the overproduction ofEPA. As is well known to one skilled in the art, various othercombinations of the following enzymatic activities may be useful toexpress in a host in conjunction with the acyltransferases describedherein: a Δ15 desaturase, a Δ4 desaturase, a Δ5 desaturase, a Δ6desaturase, a Δ17 desaturase, a Δ9 desaturase, a Δ8 desaturase and/or anelongase(s) (see FIG. 2). The particular genes included within aparticular expression cassette will depend on the host cell (and itsPUFA profile and/or desaturase profile), the availability of substrateand the desired end product(s).

Thus, within the context of the present invention, it may be useful tomodulate the expression of the TAG biosynthetic pathway by any one ofthe methods described above. For example, the present invention providesgenes encoding key enzymes in the fatty acid biosynthetic pathwayleading to the storage of TAGs. These genes encode the DGAT1 and ARE2enzymes. It will be particularly useful to modify the expression levelsof these genes in oleaginous yeasts to maximize production andaccumulation of TAGs using various means for metabolic engineering ofthe host organism. In preferred embodiments, modification of theexpression levels of these genes in combination with expression ofω-3/ω-6 biosynthetic genes can be utilized to maximize production andaccumulation of preferred PUFAs in the TAG pool.

Metabolic Engineering to Down-Regulate Undesirable Genes andBiosynthetic Pathways Affecting Fatty Acid Synthesis and OilAccumulation in Oleaginous Yeast

In some embodiments, it may be useful to disrupt or inactivate a hostorganism's native DGAT1 and/or ARE2, based on the complete sequencesdescribed herein, the complement of those complete sequences,substantial portions of those sequences, codon-optimizedacyltransferases derived therefrom, and those sequences that aresubstantially homologous thereto.

For gene disruption, a foreign DNA fragment (typically a selectablemarker gene) is inserted into the structural gene to be disrupted inorder to interrupt its coding sequence and thereby functionallyinactivate the gene. Transformation of the disruption cassette into thehost cell results in replacement of the functional native gene byhomologous recombination with the non-functional disrupted gene (see,for example: Hamilton et al., J. Bacteriol. 171:4617-4622 (1989); Balbaset al., Gene 136:211-213 (1993); Gueldener et al., Nucleic Acids Res.24:2519-2524 (1996); and Smith et al., Methods Mol. Cell. Biol.5:270-277(1996)).

Antisense technology is another method of down-regulating genes when thesequence of the target gene is known. To accomplish this, a nucleic acidsegment from the desired gene is cloned and operably linked to apromoter such that the anti-sense strand of RNA will be transcribed.This construct is then introduced into the host cell and the antisensestrand of RNA is produced. Antisense RNA inhibits gene expression bypreventing the accumulation of mRNA that encodes the protein ofinterest. The person skilled in the art will know that specialconsiderations are associated with the use of antisense technologies inorder to reduce expression of particular genes. For example, the properlevel of expression of antisense genes may require the use of differentchimeric genes utilizing different regulatory elements known to theskilled artisan.

Although targeted gene disruption and antisense technology offereffective means of down-regulating genes where the sequence is known,other less specific methodologies have been developed that are notsequence-based. For example, cells may be exposed to UV radiation andthen screened for the desired phenotype. Mutagenesis with chemicalagents is also effective for generating mutants and commonly usedsubstances include chemicals that affect nonreplicating DNA (e.g., HNO₂and NH₂OH), as well as agents that affect replicating DNA (e.g.,acridine dyes, notable for causing frameshift mutations). Specificmethods for creating mutants using radiation or chemical agents are welldocumented in the art. See, for example: Thomas D. Brock inBiotechnology: A Textbook of Industrial Microbiology, 2^(nd) ed. (1989)Sinauer Associates: Sunderland, Mass.; or Deshpande, Mukund V., Appl.Biochem. Biotechnol., 36:227 (1992).

Another non-specific method of gene disruption is the use oftransposable elements or transposons. Transposons are genetic elementsthat insert randomly into DNA but can be later retrieved on the basis ofsequence to determine where the insertion has occurred. Both in vivo andin vitro transposition methods are known. Both methods involve the useof a transposable element in combination with a transposase enzyme. Whenthe transposable element or transposon is contacted with a nucleic acidfragment in the presence of the transposase, the transposable elementwill randomly insert into the nucleic acid fragment. The technique isuseful for random mutagenesis and for gene isolation, since thedisrupted gene may be identified on the basis of the sequence of thetransposable element. Kits for in vitro transposition are commerciallyavailable [see, for example: 1.) The Primer Island Transposition Kit,available from Perkin Elmer Applied Biosystems, Branchburg, N.J., basedupon the yeast Ty1 element; 2.) The Genome Priming System, availablefrom New England Biolabs, Beverly, Mass., based upon the bacterialtransposon Tn7; and 3.) the EZ::TN Transposon Insertion Systems,available from Epicentre Technologies, Madison, Wis., based upon the Tn5bacterial transposable element].

Thus, within the context of the present invention, it may be useful todisrupt one of the acyltransferase genes of the invention. For example,it may be necessary to disrupt genes and pathways that diminish theexisting fatty acid pool and/or that hydrolyze TAGs to regulate (and/ormaximize) TAG accumulation.

Expression Systems, Cassettes and Vectors

The genes and gene products of the instant sequences described hereinmay be produced in microbial host cells, particularly in the cells ofoleaginous yeasts (e.g., Yarrowia lipolytica). Expression in recombinantmicrobial hosts may be useful for the transfer of various fatty acids toTAGs.

Microbial expression systems and expression vectors containingregulatory sequences that direct high level expression of foreignproteins are well known to those skilled in the art. Any of these couldbe used to construct chimeric genes for production of the gene productsof the instant acyltransferase sequences. These chimeric genes couldthen be introduced into appropriate microorganisms via transformation toprovide high level expression of the encoded enzymes.

Vectors or DNA cassettes useful for the transformation of suitable hostcells are well known in the art. The specific choice of sequencespresent in the construct is dependent upon the desired expressionproducts (supra), the nature of the host cell and the proposed means ofseparating transformed cells versus non-transformed cells. Typically,however, the vector or cassette contains sequences directingtranscription and translation of the relevant gene(s), a selectablemarker and sequences allowing autonomous replication or chromosomalintegration. Suitable vectors comprise a region 5′ of the gene thatcontrols transcriptional initiation and a region 3′ of the DNA fragmentthat controls transcriptional termination. It is most preferred whenboth control regions are derived from genes from the transformed hostcell, although it is to be understood that such control regions need notbe derived from the genes native to the specific species chosen as aproduction host.

Initiation control regions or promoters which are useful to driveexpression of the instant ORFs in the desired host cell are numerous andfamiliar to those skilled in the art. Virtually any promoter capable ofdirecting expression of these genes in the selected host cell issuitable for the present invention. Expression in a host cell can beaccomplished in a transient or stable fashion. Transient expression canbe accomplished by inducing the activity of a regulatable promoteroperably linked to the gene of interest. Stable expression can beachieved by the use of a constitutive promoter operably linked to thegene of interest. As an example, when the host cell is yeast,transcriptional and translational regions functional in yeast cells areprovided, particularly from the host species. The transcriptionalinitiation regulatory regions can be obtained, for example, from: 1.)genes in the glycolytic pathway, such as alcohol dehydrogenase,glyceraldehyde-3-phosphate-dehydrogenase (see U.S. patent applicationSer. No. 10/869,630, incorporated herein by reference), phosphoglyceratemutase (see U.S. patent application Ser. No. 10/869,630),fructose-bisphosphate aldolase (see U.S. Patent Application No.60/519,971, incorporated herein by reference), phosphoglucose-isomerase,phosphoglycerate kinase, glycerol-3-phosphate O-acyltransferase (seeU.S. Patent Application No. 60/610,060), etc.; or, 2.) regulatable genessuch as acid phosphatase, lactase, metallothionein, glucoamylase, thetranslation elongation factor EF1-a (TEF) protein (U.S. Pat. No.6,265,185), ribosomal protein S7 (U.S. Pat. No. 6,265,185), etc. Any oneof a number of regulatory sequences can be used, depending upon whetherconstitutive or induced transcription is desired, the efficiency of thepromoter in expressing the ORF of interest, the ease of construction andthe like.

Nucleotide sequences surrounding the translational initiation codon‘ATG’ have been found to affect expression in yeast cells. If thedesired polypeptide is poorly expressed in yeast, the nucleotidesequences of exogenous genes can be modified to include an efficientyeast translation initiation sequence to obtain optimal gene expression.For expression in yeast, this can be done by site-directed mutagenesisof an inefficiently expressed gene by fusing it in-frame to anendogenous yeast gene, preferably a highly expressed gene.Alternatively, one can determine the consensus translation initiationsequence in the host and engineer this sequence into heterologous genesfor their optimal expression in the host of interest (see, e.g., U.S.patent application Ser. No. 10/840,478 for specific teachings applicablefor Yarrowia lipolytica).

The termination region can be derived from the 3′ region of the genefrom which the initiation region was obtained or from a different gene.A large number of termination regions are known and functionsatisfactorily in a variety of hosts (when utilized both in the same anddifferent genera and species from where they were derived). Thetermination region usually is selected more as a matter of conveniencerather than because of any particular property. Preferably, thetermination region is derived from a yeast gene, particularlySaccharomyces, Schizosaccharomyces, Candida, Yarrowia or Kluyveromyces.The 3′-regions of mammalian genes encoding γ-interferon and α-2interferon are also known to function in yeast. Termination controlregions may also be derived from various genes native to the preferredhosts. Optionally, a termination site may be unnecessary; however, it ismost preferred if included.

As one of skill in the art is aware, merely inserting a gene into acloning vector does not ensure that it will be successfully expressed atthe level needed. In response to the need for a high expression rate,many specialized expression vectors have been created by manipulating anumber of different genetic elements that control aspects oftranscription, translation, protein stability, oxygen limitation andsecretion from the host cell. More specifically, some of the molecularfeatures that have been manipulated to control gene expression include:1.) the nature of the relevant transcriptional promoter and terminatorsequences; 2.) the number of copies of the cloned gene and whether thegene is plasmid-borne or integrated into the genome of the host cell;3.) the final cellular location of the synthesized foreign protein; 4.)the efficiency of translation in the host organism; 5.) the intrinsicstability of the cloned gene protein within the host cell; and 6.) thecodon usage within the cloned gene, such that its frequency approachesthe frequency of preferred codon usage of the host cell. Each of thesetypes of modifications are encompassed in the present invention, asmeans to further optimize expression of the acyltransferase enzymes.

Preferred Microbial Hosts For Recombinant Expression of Acyltransferases

Host cells for expression of the instant DGAT1 and ARE2 genes andnucleic acid fragments may include microbial hosts that grow on avariety of feedstocks, including simple or complex carbohydrates,organic acids and alcohols and/or hydrocarbons over a wide range oftemperature and pH values. Although the genes described in the instantinvention have been isolated for expression in an oleaginous yeast, andin particular Yarrowia lipolytica, it is contemplated that becausetranscription, translation and the protein biosynthetic apparatus ishighly conserved, any bacteria, yeast, algae and/or filamentous funguswill be a suitable host for expression of the present nucleic acidfragments.

Preferred microbial hosts are oleaginous organisms, such as oleaginousyeasts. These oleaginous organisms are naturally capable of oilsynthesis and accumulation, wherein the total oil content can comprisegreater than about 25% of the cellular dry weight, more preferablygreater than about 30% of the cellular dry weight and most preferablygreater than about 40% of the cellular dry weight. Additionally, thereis basis for the use of these organisms for the production of PUFAs, asseen in co-pending U.S. patent application Ser. No. 10/840,579, hereinincorporated entirely by reference.

Genera typically identified as oleaginous yeast include, but are notlimited to: Yarrowia, Candida, Rhodotorula, Rhodosporidium,Cryptococcus, Trichosporon and Lipomyces. More specifically,illustrative oil-synthesizing yeasts include: Rhodosporidium toruloides,Lipomyces starkeyii, L. lipoferus, Candida revkaufi, C. pulcherrima, C.tropicalis, C. utilis, Trichosporon pullans, T. cutaneum, Rhodotorulaglutinus, R. graminis, and Yarrowia lipolytica (formerly classified asCandida lipolytica).

Most preferred is the oleaginous yeast Yarrowia lipolytica; and, in afurther embodiment, most preferred are the Y. lipolytica strainsdesignated as ATCC #20362, ATCC #8862, ATCC #18944, ATCC #76982, ATCC#90812 and/or LGAM S(7)1 (Papanikolaou S., and Aggelis G., Bioresour.Technol. 82(1):43-9 (2002)).

Transformation of Microbial Hosts

Once the DNA encoding a polypeptide suitable for expression in anoleaginous yeast has been obtained, it is placed in a plasmid vectorcapable of autonomous replication in a host cell or it is directlyintegrated into the genome of the host cell. Integration of expressioncassettes can occur randomly within the host genome or can be targetedthrough the use of constructs containing regions of homology with thehost genome sufficient to target recombination within the host locus.Where constructs are targeted to an endogenous locus, all or some of thetranscriptional and translational regulatory regions can be provided bythe endogenous locus.

Where two or more genes are expressed from separate replicating vectors,it is desirable that each vector has a different means of selection andshould lack homology to the other construct(s) to maintain stableexpression and prevent reassortment of elements among constructs.Judicious choice of regulatory regions, selection means and method ofpropagation of the introduced construct(s) can be experimentallydetermined so that all introduced genes are expressed at the necessarylevels to provide for synthesis of the desired products.

Constructs comprising the gene of interest may be introduced into a hostcell by any standard technique. These techniques include transformation(e.g., lithium acetate transformation [Methods in Enzymology,194:186-187 (1991)]), protoplast fusion, biolistic impact,electroporation, microinjection, or any other method that introduces thegene of interest into the host cell. More specific teachings applicablefor oleaginous yeasts (i.e., Yarrowia lipolytica) include U.S. Pat. Nos.4,880,741 and 5,071,764 and Chen, D. C. et al. (Appl MicrobiolBiotechnol. 48(2):232-235-(1997)).

For convenience, a host cell that has been manipulated by any method totake up a DNA sequence (e.g., an expression cassette) will be referredto as “transformed” or “recombinant” herein. The transformed host willhave at least one copy of the expression construct and may have two ormore, depending upon whether the gene is integrated into the genome,amplified or is present on an extra-chromosomal element having multiplecopy numbers. The transformed host cell can be identified by selectionfor a marker contained on the introduced construct. Alternatively, aseparate marker construct may be co-transformed with the desiredconstruct, as many transformation techniques introduce many DNAmolecules into host cells. Typically, transformed hosts are selected fortheir ability to grow on selective media. Selective media mayincorporate an antibiotic or lack a factor necessary for growth of theuntransformed host, such as a nutrient or growth factor. An introducedmarker gene may confer antibiotic resistance, or encode an essentialgrowth factor or enzyme, thereby permitting growth on selective mediawhen expressed in the transformed host. Selection of a transformed hostcan also occur when the expressed marker protein can be detected, eitherdirectly or indirectly. The marker protein may be expressed alone or asa fusion to another protein. The marker protein can be detected by: 1.)its enzymatic activity (e.g., β-galactosidase can convert the substrateX-gal [5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside] to a coloredproduct; luciferase can convert luciferin to a light-emitting product);or 2.) its light-producing or modifying characteristics (e.g., the greenfluorescent protein of Aequorea victoria fluoresces when illuminatedwith blue light). Alternatively, antibodies can be used to detect themarker protein or a molecular tag on, for example, a protein ofinterest. Cells expressing the marker protein or tag can be selected,for example, visually, or by techniques such as FACS or panning usingantibodies. For selection of yeast transformants, any marker thatfunctions in yeast may be used. Desirably, resistance to kanamycin,hygromycin and the amino glycoside G418 are of interest, as well asability to grow on media lacking uracil or leucine.

Following transformation, substrates suitable for the gene products ofthe instant sequences (and optionally other PUFA enzymes that areexpressed within the host cell), may be produced by the host eithernaturally or transgenically, or they may be provided exogenously.

Fermentation Processes for Triacylglycerol Biosynthesis and Accumulation

The transformed microbial host cell is grown under conditions thatoptimize activity of fatty acid biosynthetic genes and acyltransferasegenes. This leads to production of the greatest and the most economicalyield of fatty acids, which can in turn be transferred to TAGs forstorage. In general, media conditions that may be optimized include thetype and amount of carbon source, the type and amount of nitrogensource, the carbon-to-nitrogen ratio, the oxygen level, growthtemperature, pH, length of the biomass production phase, length of theoil accumulation phase and the time of cell harvest. Microorganisms ofinterest, such as oleaginous yeast, are grown in complex media (e.g.,yeast extract-peptone-dextrose broth (YPD)) or a defined minimal mediathat lacks a component necessary for growth and thereby forces selectionof the desired expression cassettes (e.g., Yeast Nitrogen Base (DIFCOLaboratories, Detroit, Mich.)).

Fermentation media in the present invention must contain a suitablecarbon source. Suitable carbon sources may include, but are not limitedto: monosaccharides (e.g., glucose, fructose), disaccharides (e.g.,lactose, sucrose), oligosaccharides, polysaccharides (e.g., starch,cellulose or mixtures thereof), sugar alcohols (e.g., glycerol) ormixtures from renewable feedstocks (e.g., cheese whey permeate,cornsteep liquor, sugar beet molasses, barley malt). Additionally,carbon sources may include alkanes, fatty acids, esters of fatty acids,monoglycerides, diglycerides, triglycerides, phospholipids and variouscommercial sources of fatty acids including vegetable oils (e.g.,soybean oil) and animal fats. Additionally, the carbon substrate mayinclude one-carbon substrates (e.g., carbon dioxide, methanol,formaldehyde, formate, carbon-containing amines) for which metabolicconversion into key biochemical intermediates has been demonstrated.Hence it is contemplated that the source of carbon utilized in thepresent invention may encompass a wide variety of carbon-containingsubstrates and will only be limited by the choice of the host organism.Although all of the above mentioned carbon substrates and mixturesthereof are expected to be suitable in the present invention, preferredcarbon substrates are sugars and/or fatty acids. Most preferred isglucose and/or fatty acids containing between 10-22 carbons.

Nitrogen may be supplied from an inorganic (e.g., (NH₄)₂SO₄) or organicsource (e.g., urea, glutamate). In addition to appropriate carbon andnitrogen sources, the fermentation media must also contain suitableminerals, salts, cofactors, buffers, vitamins and other components knownto those skilled in the art suitable for the growth of the microorganismand promotion of the enzymatic pathways necessary for fatty acidproduction. Particular attention is given to several metal ions (e.g.,Mn⁺², Co+², Zn⁺², Mg⁺²) that promote synthesis of lipids and PUFAs(Nakahara, T. et al., Ind. Appl. Single Cell Oils, D. J. Kyle and R.Colin, eds. pp 61-97 (1992)).

Preferred growth media in the present invention are common commerciallyprepared media, such as Yeast Nitrogen Base (DIFCO Laboratories,Detroit, Mich.). Other defined or synthetic growth media may also beused and the appropriate medium for growth of the particularmicroorganism will be known by one skilled in the art of microbiology orfermentation science. A suitable pH range for the fermentation istypically between about pH 4.0 to pH 8.0, wherein pH 5.5 to pH 7.0 ispreferred as the range for the initial growth conditions. Thefermentation may be conducted under aerobic or anaerobic conditions,wherein microaerobic conditions are preferred.

Typically, accumulation of high levels of fatty acids and TAGs inoleaginous yeast cells requires a two-stage process, since the metabolicstate must be “balanced” between growth and synthesis/storage of fats.Thus, most preferably, a two-stage fermentation process is necessary forthe production of oils in oleaginous yeast. In this approach, the firststage of the fermentation is dedicated to the generation andaccumulation of cell mass and is characterized by rapid cell growth andcell division. In the second stage of the fermentation, it is preferableto establish conditions of nitrogen deprivation in the culture topromote high levels of lipid accumulation. The effect of this nitrogendeprivation is to reduce the effective concentration of AMP in thecells, thereby reducing the activity of the NAD-dependent isocitratedehydrogenase of mitochondria. When this occurs, citric acid willaccumulate, thus forming abundant pools of acetyl-CoA in the cytoplasmand priming fatty acid synthesis. Thus, this phase is characterized bythe cessation of cell division followed by the synthesis of fatty acidsand accumulation of TAGs.

Although cells are typically grown at about 30° C., some studies haveshown increased synthesis of unsaturated fatty acids at lowertemperatures (Yongmanitchai and Ward, Appl. Environ. Microbiol.57:419-25 (1991)). Based on process economics, this temperature shiftshould likely occur after the first phase of the two-stage fermentation,when the bulk of the organisms' growth has occurred.

It is contemplated that a variety of fermentation process designs may beapplied, where commercial production of fatty acids and TAGs using theinstant acyltransferase genes is desired. For example, commercialproduction of TAGs containing PUFAs from a recombinant microbial hostmay be produced by a batch, fed-batch or continuous fermentationprocess.

A batch fermentation process is a closed system wherein the mediacomposition is set at the beginning of the process and not subject tofurther additions beyond those required for maintenance of pH and oxygenlevel during the process. Thus, at the beginning of the culturingprocess the media is inoculated with the desired organism and growth ormetabolic activity is permitted to occur without adding additionalsubstrates (i.e., carbon and nitrogen sources) to the medium. In batchprocesses the metabolite and biomass compositions of the system changeconstantly up to the time the culture is terminated. In a typical batchprocess, cells moderate through a static lag phase to a high-growth logphase and finally to a stationary phase, wherein the growth rate isdiminished or halted. Left untreated, cells in the stationary phase willeventually die. A variation of the standard batch process is thefed-batch process, wherein the substrate is continually added to thefermentor over the course of the fermentation process. A fed-batchprocess is also suitable in the present invention. Fed-batch processesare useful when catabolite repression is apt to inhibit the metabolismof the cells or where it is desirable to have limited amounts ofsubstrate in the media at any one time. Measurement of the substrateconcentration in fed-batch systems is difficult and therefore may beestimated on the basis of the changes of measurable factors such as pH,dissolved oxygen and the partial pressure of waste gases (e.g., CO₂).Batch and fed-batch culturing methods are common and well known in theart and examples may be found in Thomas D. Brock in Biotechnology: ATextbook of Industrial Microbiology, 2^(nd) ed., (1989) SinauerAssociates: Sunderland, M A; or Deshpande, Mukund V., Appl. Biochem.Biotechnol., 36:227 (1992), herein incorporated by reference.

Commercial production of fatty acids using the instant genes may also beaccomplished by a continuous fermentation process wherein a definedmedia is continuously added to a bioreactor while an equal amount ofculture volume is removed simultaneously for product recovery.Continuous cultures generally maintain the cells in the log phase ofgrowth at a constant cell density. Continuous or semi-continuous culturemethods permit the modulation of one factor or any number of factorsthat affect cell growth or end product concentration. For example, oneapproach may limit the carbon source and allow all other parameters tomoderate metabolism. In other systems, a number of factors affectinggrowth may be altered continuously while the cell concentration,measured by media turbidity, is kept constant. Continuous systems striveto maintain steady state growth and thus the cell growth rate must bebalanced against cell loss due to media being drawn off the culture.Methods of modulating nutrients and growth factors for continuousculture processes, as well as techniques for maximizing the rate ofproduct formation, are well known in the art of industrial microbiologyand a variety of methods are detailed by Brock, supra.

Purification of Fatty Acids

Fatty acids, including PUFAs, may be found in the host microorganism asfree fatty acids or in esterified forms such as acylglycerols,phospholipids, sulfolipids or glycolipids, and may be extracted from thehost cell through a variety of means well-known in the art. One reviewof extraction techniques, quality analysis and acceptability standardsfor yeast lipids is that of Z. Jacobs (Critical Reviews in Biotechnology12(5/6):463-491 (1992)). A brief review of downstream processing is alsoavailable by A. Singh and O. Ward (Adv. Appl. Microbiol. 45:271-312(1997)).

In general, means for the purification of fatty acids (including PUFAs)may include extraction with organic solvents, sonication, supercriticalfluid extraction (e.g., using carbon dioxide), saponification andphysical means such as presses, or combinations thereof. Of particularinterest is extraction with methanol and chloroform in the presence ofwater (E. G. Bligh & W. J. Dyer, Can. J. Biochem. Physiol. 37:911-917(1959)). Where desirable, the aqueous layer can be acidified toprotonate negatively-charged moieties and thereby increase partitioningof desired products into the organic layer. After extraction, theorganic solvents can be removed by evaporation under a stream ofnitrogen. When isolated in conjugated forms, the products may beenzymatically or chemically cleaved to release the free fatty acid or aless complex conjugate of interest, and can then be subject to furthermanipulations to produce a desired end product. Desirably, conjugatedforms of fatty acids are cleaved with potassium hydroxide.

If further purification is necessary, standard methods can be employed.Such methods may include extraction, treatment with urea, fractionalcrystallization, HPLC, fractional distillation, silica gelchromatography, high-speed centrifugation or distillation, orcombinations of these techniques. Protection of reactive groups, such asthe acid or alkenyl groups, may be done at any step through knowntechniques (e.g., alkylation, iodination). Methods used includemethylation of the fatty acids to produce methyl esters. Similarly,protecting groups may be removed at any step. Desirably, purification offractions containing GLA, STA, ARA, DHA and EPA may be accomplished bytreatment with urea and/or fractional distillation.

DESCRIPTION OF PREFERRED EMBODIMENTS

The ultimate goal of the work described herein is the development of anoleaginous yeast that accumulates TAGs enriched in ω-3 and/or ω-6 PUFAs.Toward this end, acyltransferases must be identified that functionefficiently in oleaginous yeasts, to enable synthesis and highaccumulation of preferred TAGs in storage lipid pools. Specifically,modification of the expression levels of these acyltransferases willenable increased transfer of fatty acids (and particularly, PUFAs) toTAGs. Thus, identification of efficient acyltransferases is necessaryfor the manipulation of the amount of ω-3/ω-6 PUFAs incorporated intothe TAG fraction produced in host cells.

In the present invention, Applicants have isolated and cloned genes fromYarrowia lipolytica that encode DGAT1 and ARE2. This work wasundertaken, following the surprising discovery that the Y. lipolyticaDGAT2 and PDAT are only partially responsible for oil biosynthesis(based on analysis of a double knockout mutant; see co-pending U.S.patent application Ser. No. 10/882,760). Confirmation of the DGAT1gene's activity was provided herein based upon lower oil content (totalfatty acids as a % of dry cell weight) in Yarrowia strains whereindisruption of the native DGAT1 had occurred by targeted gene replacementthrough homologous recombination (Example 9). Additionally,over-expression of the DGAT1 and ARE2 genes of the invention areexpected to increase oil content (total fatty acids as a % of dry cellweight), based on results obtained wherein the Yarrowia DGAT1 wasoverexpressed (Example 12).

Following identification of the Y. lipolytica DGAT1 (SEQ ID NOs:13 and14), a proprietary database of Mortierella alpina cDNA sequences wassearched to identify an orthologous gene(s). This lead to theidentification of a DGAT1 homolog, whose full nucleotide sequence wasthus determined (SEQ ID NO:17). Based on these two novel sequences, theApplicants were then able to identify a suite of unique fungal DGAT1orthologs that are involved in the synthesis of TAGs. More specifically,these DGAT1s have been identified from Neurospora crassa (SEQ ID NO:19),Gibberella zeae PH-1 (SEQ ID NO:20), Magnaporthe grisea (SEQ ID NO:21)and Aspergillus nidulans (SEQ ID NO:22), based on comparison of theYarrowia and Mortierella DNA sequences to the GenBank database using theBLAST algorithm, well known to those skilled in the art.

Analysis of these six DGAT1 sequences, when aligned with DGAT1 sequencesfrom mouse (SEQ ID NO:169), soy (SEQ ID NO:170), Arabidopsis (SEQ IDNO:171), rice (SEQ ID NO:172), Perilla (SEQ ID NO:173) and wheat (SEQ IDNO:174), however, reveals unique characteristics that are specific forfungi and absent in DGAT1s from the non-fungal organisms described above(i.e., fungal motifs #1-8, herein described as SEQ ID NOs:23-30).Furthermore, in a broader context, common motifs universally found inDGAT1 enzymes were also discovered (SEQ ID NOs:31-37). These uniquefungal and universal conserved domains are between amino acids 97-105,278-284, 334-341, 364-374, 418-424, 415-424, 456-466 and 513-519, withrespect to SEQ ID NO:14. As is well known to one of skill in the art,these motifs will therefore be diagnostic for DGAT1s and will permitrapid identification of novel DGAT1s. The motifs described herein are tobe distinguished from the ‘FxxPxYR’ motif (SEQ ID NO:38) that wasrecently described by Lardizabal et al. (US04/0107459 A1), which ispreferably useful to identify DGAT2s of fungal origin.

The Applicants conclude that these DGAT1 and ARE2 acyltransferase genesare useful for expression in various microbial hosts, and particularlyfor over-expression in oleaginous yeasts (e.g., Yarrowia lipolytica).Additional benefits may result, since expression of the acyltransferasescan also be put under the control of strong constitutive or regulatedpromoters that do not have the regulatory constraints of the nativegene.

EXAMPLES

The present invention is further defined in the following Examples. Itshould be understood that these Examples, while indicating preferredembodiments of the invention, are given by way of illustration only.From the above discussion and these Examples, one skilled in the art canascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

General Methods

Standard recombinant DNA and molecular cloning techniques used in theExamples are well known in the art and are described by: 1.) Sambrook,J., Fritsch, E. F. and Maniatis, T. Molecular Cloning: A LaboratoryManual; Cold Spring Harbor Laboratory: Cold Spring Harbor, N.Y. (1989)(Maniatis); 2.) T. J. Silhavy, M. L. Bennan, and L. W. Enquist,Experiments with Gene Fusions; Cold Spring Harbor Laboratory: ColdSpring Harbor, N.Y. (1984); and 3.) Ausubel, F. M. et al., CurrentProtocols in Molecular Biology, published by Greene Publishing Assoc.and Wiley-Interscience (1987).

Materials and methods suitable for the maintenance and growth ofmicrobial cultures are well known in the art. Techniques suitable foruse in the following Examples may be found as set out in Manual ofMethods for General Bacteriology (Phillipp Gerhardt, R. G. E. Murray,Ralph N. Costilow, Eugene W. Nester, Willis A. Wood, Noel R. Krieg andG. Briggs Phillips, Eds), American Society for Microbiology: Washington,D.C. (1994); or by Thomas D. Brock in Biotechnology: A Textbook ofIndustrial Microbiologv, 2^(nd) ed., Sinauer Associates: Sunderland,Mass. (1989). All reagents, restriction enzymes and materials used forthe growth and maintenance of microbial cells were obtained from AldrichChemicals (Milwaukee, Wis.), DIFCO Laboratories (Detroit, Mich.),GIBCO/BRL (Gaithersburg, Md.) or Sigma Chemical Company (St. Louis,Mo.), unless otherwise specified.

E. coli TOP10 cells and E. coli Electromax DH10B cells were obtainedfrom Invitrogen (Carlsbad, Calif.). Max Efficiency competent cells of E.coli DH5a were obtained from GIBCO/BRL (Gaithersburg, Md.). E. coli(XL1-Blue) competent cells were purchased from the Stratagene Company(San Diego, Calif.). E. coli strains were typically grown at 37° C. onLuria Bertani (LB) plates. General molecular cloning was performedaccording to standard methods (Sambrook et al., supra). Oligonucleotideswere synthesized by Sigma-Genosys (Spring, Tex.).

All polymerase chain reactions (PCRs) were performed in a thermocylerusing DNA polymerase in a buffer recommended by the manufacturer of thepolymerase. Unless specified otherwise, amplification was carried out asfollows: initial denaturation at 95° C. for 1 min, followed by 30 cyclesof denaturation at 95° C. for 30 sec, annealing at 55° C. for 1 min, andelongation at 72° C. for 1 min. A final elongation cycle at 72° C. for10 min was carried out, followed by reaction termination at 4° C. PCRproducts were cloned into Promega's pGEM-T-easy vector (Madison, Wis.),unless otherwise noted.

DNA sequence was generated on an ABI Automatic sequencer using dyeterminator technology (U.S. Pat. No. 5,366,860; EP 272,007) using acombination of vector and insert-specific primers. Sequence editing wasperformed in Sequencher (Gene Codes Corporation, Ann Arbor, Mich.). Allsequences represent coverage at least two times in both directions.Comparisons of genetic sequences were accomplished using DNASTARsoftware (DNASTAR, Inc., Madison, Wis.). The percent identities betweenthese proteins were determined by the Megalign program of DNASTAR usingClustal W with the following parameters: gap penalty=10, gap lengthpenalty=0.2, delay divergent seqs (%)=30, DNA transition weight=0.5 andprotein weight matrix by Gonnet series.

The meaning of abbreviations is as follows: “sec” means second(s), “min”means minute(s), “h” means hour(s), “d” means day(s), “μl” meansmicroliter(s), “mL” means milliliter(s), “L” means liter(s), “μM” meansmicromolar, “mM” means millimolar, “M” means molar, “mmol” meansmillimole(s), “μmole” mean micromole(s), “g” means gram(s), “μg” meansmicrogram(s), “ng” means nanogram(s), “U” means unit(s), “bp” means basepair(s) and “kB” means kilobase(s).

Transformation and Cultivation of Yarrowia lipolytica

Yarrowia lipolytica strains ATCC #20362, #76982 and #90812 werepurchased from the American Type Culture Collection (Rockville, Md.). Y.lipolytica strains were usually grown at 28° C. on YPD agar (1% yeastextract, 2% bactopeptone, 2% glucose, 2% agar).

Transformation of Yarrowia lipolytica was performed according to themethod of Chen, D. C. et al. (Appl. Microbiol. Biotechnol. 48(2):232-235(1997)), unless otherwise noted. Briefly, Yarrowia was streaked onto aYPD plate and grown at 30° C. for approximately 18 hr. Several largeloopfuls of cells were scraped from the plate and resuspended in 1 mL oftransformation buffer containing: 2.25 mL of 50% PEG, average MW 3350;0.125 mL of 2 M L1 acetate, pH 6.0; 0.125 mL of 2 M DTT; and 50 μgsheared salmon sperm DNA. Then, approximately 500 ng of linearizedplasmid DNA was incubated in 100 μl of resuspended cells, and maintainedat 39° C. for 1 hr with vortex mixing at 15 min intervals. The cellswere plated onto selection media plates and maintained at 30° C. for 2to 3 days.

For selection of transformants, minimal medium (“MM”) was generallyused; the composition of MM is as follows: 0.17% yeast nitrogen base(DIFCO Laboratories, Detroit, Mich.) without ammonium sulfate or aminoacids, 2% glucose, 0.1% proline, pH 6.1). Supplements of adenine,leucine, lysine and/or uracil were added as appropriate to a finalconcentration of 0.01% (thereby producing “MMA”, “MMLe”, “MMLy” and“MMU” selection media, each prepared with 20 g/L agar).

Alternatively, transformants were selected on 5-fluoroorotic acid(“FOA”; also 5-fluorouracil-6-carboxylic acid monohydrate) selectionmedia, comprising: 0.17% yeast nitrogen base (DIFCO Laboratories,Detroit, Mich.) without ammonium sulfate or amino acids, 2% glucose,0.1% proline, 75 mg/L uracil, 75 mg/L uridine, 900 mg/L FOA (ZymoResearch Corp., Orange, Calif.) and 20 g/L agar.

To promote oleaginous conditions, High Glucose Media (“HGM”) wasprepared as follows: 14 g/L KH₂PO₄, 4 g/LK₂HPO₄, 2 g/L MgSO₄ 7H₂O, 80g/L glucose (pH 6.5).

Fatty Acid Analysis of Yarrowia lipolytica

For fatty acid analysis, cells were collected by centrifugation andlipids were extracted as described in Bligh, E. G. & Dyer, W. J. (Can.J. Biochem. Physiol. 37:911-917 (1959)). Fatty acid methyl esters wereprepared by transesterification of the lipid extract with sodiummethoxide (Roughan, G., and Nishida I. Arch Biochem Biophys.276(1):38-46 (1990)) and subsequently analyzed with a Hewlett-Packard6890 GC fitted with a 30-m×0.25 mm (i.d.) HP-INNOWAX (Hewlett-Packard)column. The oven temperature was from 170° C. (25 min hold) to 185° C.at 3.5° C./min.

For direct base transesterification, Yarrowia culture (3 mL) washarvested, washed once in distilled water, and dried under vacuum in aSpeed-Vac for 5-10 min. Sodium methoxide (100 μl of 1%) was added to thesample, and then the sample was vortexed and rocked for 20 min. Afteradding 3 drops of 1 M NaCl and 400 μl hexane, the sample was vortexedand spun. The upper layer was removed and analyzed by GC as describedabove.

Example 1 Construction of Plasmids Suitable for Gene Expression inYarrowia lipolytica

The present Example describes the construction of plasmids pY5, pY5-13,pY20 and pLV5.

Construction of Plasmid pY5

The plasmid pY5, a derivative of pINA532 (a gift from Dr. ClaudeGaillardin, Insitut National Agronomics, Centre de biotechnologieAgro-Industrielle, laboratoire de Genetique Moleculaire et CellularieINRA-CNRS, F-78850 Thiverval-Grignon, France), was constructed forexpression of heterologous genes in Yarrowia lipolytica, as diagrammedin FIG. 3.

First, the partially-digested 3598 bp EcoRI fragment containing theARS18 sequence and LEU2 gene of pINA532 was subcloned into the EcoRIsite of pBluescript (Strategene, San Diego, Calif.) to generate pY2. TheTEF promoter (Muller S. et al., Yeast, 14:1267-1283 (1998)) wasamplified from Y. lipolytica genomic DNA by PCR using TEF5′ (SEQ IDNO:39) and TEF3′ (SEQ ID NO:40) as primers. PCR amplification wascarried out in a 50 μl total volume containing: 100 ng Yarrowia genomicDNA, PCR buffer containing 10 mM KCl, 10 mM (NH₄)₂SO₄, 20 mM Tris-HCl(pH 8.75), 2 mM MgSO₄, 0.1% Triton X-100, 100 μg/mL BSA (finalconcentration), 200 μM each deoxyribonucleotide triphosphate, 10 pmoleof each primer and 1 μl of PfuTurbo DNA polymerase (Stratagene).Amplification was carried out as follows: initial denaturation at 95° C.for 3 min, followed by 35 cycles of the following: 95° C. for 1 min, 56°C. for 30 sec, 72° C. for 1 min. A final extension cycle of 72° C. for10 min was carried out, followed by reaction termination at 4° C. The418 bp PCR product was ligated into pCR-Blunt to generate pIP-tef. TheBamHI/EcoRVfragment of pIP-tef was subcloned into the BamHI/SmaI sitesof pY2 to generate pY4.

The XPR2 transcriptional terminator was amplified by PCR using pINA532as template and XPR5′ (SEQ ID NO:41) and XPR3′ (SEQ ID NO:42) asprimers. The PCR amplification was carried out in a 50 μl total volume,using the components and conditions described above. The 179 bp PCRproduct was digested with SacII and then ligated into the SacII site ofpY4 to generate pY5. Thus, pY5 (shown in FIG. 3) is useful as aYarrowia-E. coli shuttle plasmid containing: a Yarrowia autonomousreplication sequence (ARS18); a ColE1 plasmid origin of replication; anampicillin-resistance gene (AmpR) for selection in E. coli; a YarrowiaLEU2 gene (E.C. 1.1.1.85, encoding isopropylmalate isomerase) forselection in Yarrowia; the translation elongation promoter (TEF) forexpression of heterologous genes in Yarrowia; and the extracellularprotease gene terminator (XPR2) for transcriptional termination ofheterologous gene expression in Yarrowia.

Construction of Plasmid pY5-13

pY5-13 was constructed as a derivative of pY5 to faciliate subcloningand heterologous gene expression in Yarrowia lipolytica. Specifically,pY5-13 was constructed by 6 rounds of site-directed mutagenesis usingpY5 as template. Both SalI and ClaI sites were eliminated from pY5 bysite-directed mutagenesis using oligonucleotides YL5 and YL6 (SEQ IDNOs:43 and 44) to generate pY5-5. A SalI site was introduced into pY5-5between the LEU2 gene and the TEF promoter by site-directed mutagenesisusing oligonucleotides YL9 and YL10 (SEQ ID NOs:45 and 46) to generatepY5-6. A PacI site was introduced into pY5-6 between the LEU2 gene andARS18 using oligonucleotides YL7 and YL8 (SEQ ID NOs:47 and 48) togenerate pY5-8. A NcoI site was introduced into pY5-8 around thetranslation start codon of the TEF promoter using oligonucleotides YL3and YL4 (SEQ ID NOs:49 and 50) to generate pY5-9. The NcoI site insidethe LEU2 gene of pY5-9 was eliminated using YL1 and YL2 oligonucleotides(SEQ ID NOs:51 and 52) to generate pY5-12. Finally, a BsiWI site wasintroduced into pY5-12 between the ColE1 and XPR2 region usingoligonucleotides YL61 and YL62 (SEQ ID NOs:53 and 54) to generatepY5-13.

Construction of Plasmids pY20 and pLV5

Plasmid pY20 (SEQ ID NO:55) is a derivative of pY5. It was constructedby inserting a Not I fragment containing a chimeric hygromycinresistance gene into the Not I site of pY5. Specifically, the E. colihygromycin resistance gene (SEQ ID NO:56; “HPT”; Kaster, K. R. et al.,Nucleic Acids Res. 11:6895-6911 (1983)) was PCR amplified forexpression. The chimeric gene had the hygromycin resistance ORF underthe control of the Y. lipolytica TEF promoter.

Plasmid pLV5 is a derivative of pY20. It was constructed by replacingthe hygromycin resistant gene with the Yarrowia Ura3 gene. A 1.7 kB DNAfragment (SEQ ID NO:58) containing the Yarrowia Ura3 gene was PCRamplified using oligonucleotides KU5 and KU3 (SEQ ID NOs:60 and 61) asprimers and Yarrowia genomic DNA as template.

Example 2 Cloning of a Partial Yarrowia lipolyticaAcyl-CoA:Diacylglycerol Acyltransferase (DGAT2) Gene and Disruption ofthe Endogenous DGAT2 Gene

The present Example describes the use of degenerate PCR primers toisolate a partial coding sequence of the Yarrowia lipolytica DGAT2 andthe use of the partial sequence to disrupt the native gene in Y.lipolytica.

Cloning of a Partial Putative DGAT2 Sequence From Yarrowia lipolytica byPCR Using Degenerate PCR Primers and Chromosome Walking

Genomic DNA was isolated from Y. lipolytica (ATCC #76982) using a DNeasyTissue Kit (Qiagen, Catalog #69504) and resuspended in kit buffer AE ata DNA concentration of 0.5 μg/μl. PCR amplifications were performedusing the genomic DNA as template and several sets of degenerate primersdesigned to encode conserved amino acid sequences among different knownDGAT2s (i.e., GenBank Accession Nos. NC_(—)001147 [Saccharomycescerevisiae] and AF391089 and AF391090 [Mortierella ramanniana]). Thebest results were obtained with degenerate primers P7 and P8, as shownin the Table below. TABLE 4 Degenerate Primers Used For Amplification OfA Partial Putative DGAT2 Degenerate Corresponding Primer NucleotideAmino Acid Set Description Sequence Sequence P7 (32) 5′-AACTACATCTTCGNYIFGYHPHG 29-mers GCTAYCAYCCNCAYG (SEQ ID NO:63) G-3′ (SEQ ID NO:62) P8(48) 5′-AGGGACTCGGAGG complementary to 29-mers CGCCGCCNCANACDAIVVGGASESL T-3′ (SEQ ID NO:65) (SEQ ID NO:64)[Note:Abbreviations are standard for nucleotides and proteins. The nucleicacid degeneracy code used is as follows: Y = C/T; D = A/G/T; and N= A/C/G/T.]

The PCR was carried out in a RoboCycler Gradient 40 PCR machine(Stratagene) using the manufacturer's recommendations and Accuprime Taqpolymerase (Invitrogen). Amplification was carried out as described inthe General Methods.

The expected PCR product (ca. 264 bp) was detected by 4% NuSieve (FMC)agarose gel electrophoresis, isolated, purified, cloned into the TOPO®cloning vector (Invitrogen) and sequenced. The resultant sequence(contained within SEQ ID NO:1) had homology to known DGAT2s, based onBLAST program analysis (Basic Local Alignment Search Tool; Altschul, S.F. et al., J. Mol. Biol. 215:403-410 (1993)).

Using the 264 bp fragment as an initiation point, a 673 bp fragment wasobtained by chromosome walking using the TOPO® Walker Kit (Invitrogen,Catalog #K8000-01). The chromosome walking was carried out in 6 steps,as described briefly below:

-   -   1.) Genomic DNA (5 μg) was digested with restriction enzymes Pst        I or Sac I, leaving a 3′ overhang;    -   2.) Digested DNA was treated with 0.1 U calf intestinal alkaline        phosphatase to dephosphorylate DNA;    -   3.) Primer extension was performed, using the DGAT2 specific        primer P80 (SEQ ID NO:66) and Taq polymerase;    -   4.) TOPO® Linker (1 μl) was added and the reaction was incubated        at 37° C. for 5 min to ligate TOPO® Linker to the DNA;    -   5.) PCR was performed using the DGAT2 gene specific primer, P81        (SEQ ID NO:67) and LinkAmp primer 1 (SEQ ID NO:68); and    -   6.) The newly amplified fragment was sequenced with primer P81        and LinkAmp primer 1.        The sequence of the 673 bp fragment obtained by chromosome        walking also showed homology to known DGAT2 sequences.        Targeted Disruption of the Yarrowia lipolytica DGAT2 Gene

Targeted disruption of the DGAT2 gene in Y. lipolytica ATCC #90812 andATCC #76982 was carried out by homologous recombination-mediatedreplacement of the endogenous DGAT2 gene with a targeting cassettedesignated as plasmid pY21 DGAT2. pY21 DGAT2 was derived from plasmidpY20 (Example 1; SEQ ID NO:55). Specifically, pY21 DGAT2 was created byinserting a 570 bp Hind III/Eco RI fragment into similarly linearizedpY20. The 570 bp DNA fragment contained (in 5′ to 3′ orientation): 3′homologous sequence from position +1090 to +1464 (of the coding sequence(ORF) in SEQ ID NO:1), a Bgl II restriction site and 5′ homologoussequence from position+906 to +1089 (of the coding sequence (ORF) shownin SEQ ID NO:1). The fragment was prepared by PCR amplification of 3′and 5′ sequences from the 673 bp DGAT2 PCR product obtained bychromosome walking using two pairs of PCR primers, P95 and P96 (SEQ IDNOs:69 and 70), and P97 and P98 (SEQ ID NOs:71 and 72), respectively.

pY21DGAT2 was linearized by Bgl II restriction digestion and transformedinto mid-log phase Y. lipolytica ATCC #90812 and ATCC #76982 cells, asdescribed in the General Methods. The cells were plated onto YPDhygromycin selection plates and maintained at 30° C. for 2 to 3 days.

Four Y. lipolytica ATCC #76982 hygromycin-resistant colonies andfourteen Y. lipolytica ATCC #90812 hygromycin-resistant colonies wereisolated and screened for targeted disruption by PCR. One set of PCRprimers (P115 and P116 [SEQ ID NOs:73 and 74, respectively]) wasdesigned to amplify a specific junction fragment following homologousrecombination. Another pair of PCR primers (P115 and P112 [SEQ IDNO:75]) was designed to detect the native gene.

All (4 of 4) of the hygromycin-resistant colonies of ATCC #76982 strainswere positive for the junction fragment and negative for the nativefragment; and, 2 of the 14 hygromycin-resistant colonies of ATCC #90812strains were positive for the junction fragment and negative for thenative fragment. Thus, targeted integration was confirmed in these 6strains. Disruption of the gene was further confirmed by GC analysis oftotal lipids of one of the disrupted strains, designated as “S-D2” (seeExample 9).

Example 3 Cloning of a Partial Yarrowia lipolyticaPhospholipid:Diacylglycerol Acyltransferase (PDAT) Gene and Disruptionof the Endogenous PDAT Gene

The present Example describes the use of degenerate PCR primers toisolate a partial coding sequence of Y. lipolytica PDAT and the use ofthe partial sequence to disrupt the native gene in Y. lipolytica.

Cloning of a Partial Putative PDAT Sequence from Yarrowia lipolytica ByPCR Using Degenerate PCR Primers and Chromosome Walking

Genomic DNA was isolated from Y. lipolytica (ATCC #76982) using a DNeasyTissue Kit (Qiagen, Catalog #69504) and resuspended in kit buffer AE ata DNA concentration of 0.5 μg/μl. PCR amplifications were performedusing genomic DNA as the template and several pairs of degenerateprimers encoding conserved amino acid sequences in different known PDATs(GenBank Accession Nos. NP 190069 and AB006704 [(gi:2351069Arabidopsisthaliana], and NP_(—)596330 [Schizosaccharomyces pombe]; and theSaccharomyces cerevisiae Lro 1 gene [Dahlqvist et al., Proc. Natl. Acad.Sci. USA 97:6487 (2000)]). The best results were obtained withdegenerate primers P26 and P27, as shown in the Table below. TABLE 5Degenerate Primers Used For Amplification Of A Partial Putative PDATDegenerate Corresponding Primer Nucleotide Amino Acid Set DescriptionSequence Sequence P26 (32) 5′-ATGCTGGACAAGG MLDKETGLDP 29-mersAGACCGGNCTNGAYC (SEQ ID NO:77) C-3′ (SEQ ID NO:76) P27 (16)5′-CCAGATGACGTCG complementary to 33-mers CCGCCCTTGGGNARCA SMLPKGGEVIWTNGA-3′ (SEQ ID NO:79) (SEQ ID NO:78)[Note:Abbreviations are standard for nucleotides and proteins. The nucleicacid degeneracy code used is as follows: R = A/G; Y = C/T; and N= A/C/G/T.]

The PCR was carried out in a RoboCycler Gradient 40 PCR machine(Stratagene), using the amplification conditions described in theGeneral Methods. The expected PCR product (ca. 600 bp) was detected by4% NuSieve (FMC) agarose gel electrophoresis, isolated, purified, clonedinto the TOPO® cloning vector (Invitrogen) and sequenced. The resultantsequence (contained within SEQ ID NO:7) had homology to known PDATs,based on BLAST program analysis (Altschul, S. F. et al., J. Mol. Biol.215:403-410 (1993)).

Targeted Disruption of Yarrowia lipolytica PDAT Gene

Following the sequencing of this ca. 600 bp partial coding region forPDAT, a larger DNA fragment encoding this sequence was discovered in thepublic Y. lipolytica protein database of the “Yeast project Genolevures”(Center for Bioinformatics, LaBR1, Talence Cedex, France) (see alsoDujon, B. et al., Nature 430 (6995):35-44 (2004)). This allowedisolation of a 1008 bp genomic DNA fragment comprising a portion of thePDAT gene from Y. lipolytica ATCC #90812 using PCR primers P39 and P42(SEQ ID NOs:80 and 81).

Targeted disruption of the PDAT gene in Y. lipolytica ATCC #90812 wascarried out by homologous recombination-mediated replacement of theendogenous PDAT gene with a targeting cassette designated as pLV13 (SEQID NO:82). pLV13 was derived from plasmid pLV5 (Example 1).Specifically, pLV13 was created by inserting a 992 bp Bam HI/Eco RIfragment into similarly linearized pLV5. The 992 bp DNA fragmentcontained (in 5′ to 3′ orientation): 3′ homologous sequence fromposition+877 to +1371 (of the coding sequence (ORF) in SEQ ID NO:7), aBgl II restriction site and 5′ homologous sequence from position +390 to+876 (of the coding sequence (ORF) in SEQ ID NO:7). The fragment wasprepared by PCR amplification of 3′ and 5′ sequences from the 1008 bpPCR product described above, using PCR primers P39 and P41 (SEQ IDNOs:80 and 83) and P40 and P42 (SEQ ID NOs:84 and 81), respectively.

pLV13 was linearized by Bgl II restriction digestion and was transformedinto mid-log phase Y. lipolytica ATCC #90812 cells by the lithiumacetate method (General Methods). The cells were plated onto Bio101DOB/CSM-Ura selection plates and maintained at 30° C. for 2 to 3 days.

Ten Y. lipolytica ATCC #90812 colonies were isolated and screened fortargeted disruption by PCR. One set of PCR primers (P51 and P52 [SEQ IDNOs:85 and 86, respectively]) was designed to amplify the targetingcassette. Another set of PCR primers (P37 and P38 [SEQ ID NOs:87 and 88,respectively]) was designed to detect the native gene. Ten of the tenstrains were positive for the junction fragment and 3 of the strainswere negative for the native fragment, thus confirming successfultargeted integration in these 3 strains. Disruption of the gene wasfurther confirmed by GC analysis of total lipids in one of the disruptedstrains, designated as “S-P” (see Example 9).

Example 4 Construction of a Yarrowia lipolytica Double Knockout StrainContaining Disruptions in Both PDAT and DGAT2 Genes

The present Example describes the creation of a double knockout strainthat was disrupted in both PDAT and DGAT2 genes.

Specifically, the Y. lipolytica ATCC #90812 hygromycin-resistant “S-D2”mutant (containing the DGAT2 disruption from Example 2) was transformedwith plasmid pLV13 (from Example 3) and transformants were screened byPCR, as described in Example 3. Two of twelve transformants wereconfirmed to be disrupted in both the DGAT2 and PDAT genes. Disruptionof the gene was further confirmed by GC analysis of total lipids in oneof the disrupted strains, designated as “S-D2-P” (see Example 9).

Example 5 Cloning of Full-Length Yarrowia lipolytica DGAT2 and PDATGenes

The present Example describes the recovery of the genomic sequencesflanking the disrupted DGAT2 and PDAT genes by plasmid rescue, using thesequence in the rescued plasmid to PCR the intact ORF of the nativegene. The full-length genes and their deduced amino acid sequences arecompared to other fungal DGAT2 and PDAT sequences, respectively.

Plasmid Rescue of Yarrowia lipolytica DGAT2 and PDAT Genes

Since the acyltransferase genes were disrupted by the insertion of theentire pY21DGAT2 and pLV13 vectors that each contained an E. coliampicillin-resistant gene and E. coli ori, it was possible to rescue theflanking PDAT and DGAT2 sequences in E. coli. For this, genomic DNA ofY. lipolytica strain “S-D2” (carrying the disrupted DGAT2 gene; Example2) and Y. lipolytica strain “S-P” (carrying the disrupted PDAT gene;Example 3) was isolated using the DNeasy Tissue Kit. Specifically, 10 μgof the genomic DNA was digested with 50 U of the following restrictionenzymes in a reaction volume of 200 μl: for DGAT2—Age I and Nhe I; forPDAT—Kpn I, Pac I and Sac I. Digested DNA was extracted withphenol:chloroform and resuspended in 40 μl deionized water. The digestedDNA (10 μl) was self-ligated in a 200 μl ligation mixture containing 3 UT4 DNA ligase. Each ligation reaction was carried out at 16° C. for 12hrs. The ligated DNA was extracted with phenol:chloroform andresuspended in 40 μl deionized water. Finally, 1 μl of the resuspendedligated DNA was used to transform E. coli by electroporation and platedon LB containing ampicillin (Ap). Ap-resistant transformants wereisolated and analyzed for the presence of plasmids. The following insertsizes were found in the recovered or rescued plasmids (Tables 5 and 6):TABLE 6 Insert Sizes Of Recovered DGAT2 Plasmids, According ToRestriction Enzyme Enzyme plasmid insert size (kB) AgeI 2.3 NheI 9.5

TABLE 7 Insert Sizes Of Recovered PDAT Plasmids, According ToRestriction Enzyme Enzyme plasmid insert size (kB) Kpn I 6.9 Sac I 5.4Sph I 7.0

Sequencing of the DGAT2 rescued plasmids was initiated with sequencingprimers P79 (SEQ ID NO:89) and P95 (SEQ ID NO:69). In contrast,sequencing of the PDAT plasmids was initiated with sequencing primersP84 (SEQ ID NO:90) and P85 (SEQ ID NO:91).

Based on the sequencing results, a full-length gene encoding the Y.lipolytica DGAT2 gene was assembled (2119 bp; SEQ ID NO:1).Specifically, the sequence encoded an open reading frame (ORF) of 1545bases (nucleotides +291 to +1835 of SEQ ID NO:1), while the deducedamino acid sequence was 514 residues in length (SEQ ID NO:2). Since thisORF has an initiation codon (‘ATG’) at position 1, as well as atpositions 56 and 160, it contains at least two additional nested(smaller) ORFs. Specifically, one ORF is 1380 bases long (nucleotides+456 to +1835 of SEQ ID NO:1, corresponding to SEQ ID NO:3), with adeduced amino acid sequence of 459 residues (SEQ ID NO:4); another ORFis 1068 bases long (nucleotides +768 to +1835 of SEQ ID NO:1,corresponding to SEQ ID NO:5) with a deduced amino acid sequence of 355residues (SEQ ID NO:6).

The ORF encoded by SEQ ID NO:5 has a high degree of similarity to otherknown DGAT2 enzymes and because disruption in SEQ ID NO:5 eliminated DAGAT function of the native gene (see Example 9), the polypeptide of SEQID NO:6 has been identified as clearly having DGAT2 functionality.

Following sequencing and analysis of the DGAT2 protein described above,a Yarrowia lipolytica DGAT2 protein sequence was published within thepublic Y. lipolytica protein database of the “Yeast project Genolevures”(sponsored by the Center for Bioinformatics, LaBR1, bâtiment Δ30,Universite Bordeaux 1, 351, cours de la Libération, 33405 Talence Cedex,France) (see also Dujon, B. et al., Nature 430 (6995):35-44 (2004)).Specifically, the sequence disclosed therein was identified as ORFYALI-CDS2240.1, encoding 514 amino acids, and the protein was reportedto share some similarities with tr|Q08650 Saccharomyces cerevisiaeYOR245C DGA1 acyl-CoA:diacylglycerol acyltransferase.

In a manner similar to that used to deduce the full-length sequence ofDGAT2, a full-length gene encoding the Y. lipolytica PDAT gene wasassembled (2326 bp; SEQ ID NO:7) based on sequencing results.Specifically, the sequence encoded an open reading frame of 1944 bases(nucleotides +274 to +2217 of SEQ ID NO:7), while the deduced amino acidsequence was 648 residues in length (SEQ ID NO:8).

Following sequencing and analysis of the PDAT protein described above,the Yarrowia lipolytica PDAT protein sequence was published as part ofthe public Y. lipolytica protein database of the “Yeast projectGenolevures” (supra). The PDAT sequence disclosed therein was identifiedas ORF YALI-CDS1359.1, encoding 648 amino acids, and the protein wasreported to share some similarities to sp|P40345 Saccharomycescerevisiae YNRO08w LRO1, a lecithin cholesterol acyltransferase-likegene which mediates diacylglycerol esterification.

Example 6 Identification of Additional Putative Yarrowia lipolytica DAGATs

In order to identify additional DAG ATs in Yarrowia, the public Y.lipolytica protein database of the “Yeast project Genolevures” (supra)was searched using the Saccharomyces cerevisiae ARE1 (Sc ARE1; GenBankAccession No. CAA42296) and ARE2 (Sc ARE2; GenBank Accession No. P53629)protein sequences (Yang, H. et al., Science. 272(5266):1353-1356(1996)). Both searches identified the following Y. lipolytica ORFs asthe first and second hits, respectively:

-   -   (1) YALI-CDS2011.1: annotated as “similar to spIP53629        Saccharomyces cerevisiae YNRO19w ARE2 acyl-CoA sterol        acyltransferase, hypothetical start”; 543 amino acids in length        (SEQ ID NOs:9 and 10); and    -   (2) YALI-CDS2141.1: annotated as “unnamed protein product;        weakly similar to tr|Q9FUL6 Perilla frutescens Diacylglycerol        acyltransferase (Pf DGAT1), hypothetical start”; 526 amino acids        in length (SEQ ID NOs:11 and 12).

The percent identities between these proteins were determined by theMegalign program of DNASTAR using Clustal W according to the parametersdescribed in the General Methods. The percent (%) identities are shownbelow, wherein the % identity is defined as percentage of amino acidsthat are identical between the two proteins: TABLE 8 Percent IdentitiesBetween Known Acyltransferases And Yarrowia lipolytica ORFs Sc ARE1 ScARE2 Pf DGAT1 YALI-CDS2141.1 16.6 14.5 29.5 YALI-CDS2011.1 32.6 33.818.4Based on this comparison, YALI-CDS2141.1 and YALI-CDS2011.1 (designatedherein as “YI DGAT1” and “YI ARE2”, respectively) were candidates ORFsthat were likely to encode proteins having DAG AT functionality inYarrowia.

Following the analysis of the proteins described above, the Yarrowialipolytica strain CLIB99 complete genome was published in GenBank aspart of the Genolevures project. Thus, the ORF identified asYALI-CDS2011.1 corresponds to GenBank Accession No. NC_(—)006072,locus_tag=“YALI0F06578g” and the ORF identified as YALI-CDS2141.1corresponds to GenBank Accession No. CR382130, locus_tag=“YALI0D07986g”.

Example 7 Cloning of a Yarrowia lipolytica Acyl-CoA:DiacylglycerolAcyltransferase (DGAT1) Gene and Disruption of the Endogenous DGAT1 Gene

The present Example describes the use of degenerate PCR primers toisolate the full-length coding sequence of the Yarrowia lipolytica DGAT1(encoded by ORF YALI-CDS2011.1 (Example 6)) and the use of the sequenceto disrupt the native gene in Y. lipolytica.

Cloning of a Putative DGAT1 Sequence From Yarrowia lipolytica by PCRUsing Degenerate PCR Primers

The full-length YI DGAT1 ORF was cloned by PCR using degenerate PCRprimers P201 and P203 (SEQ ID NOs:92 and 93, respectively) and Y.lipolytica ATCC #76982 (from Example 2) genomic DNA as template. Thedegenerate primers were required, since the nucleotide sequence encodingYI DGAT1 was not known.

The PCR was carried out in a RoboCycler Gradient 40 PCR machine, usingthe components and thermocycler conditions described in the GeneralMethods. The expected PCR product (ca. 1.6 kB) was detected by agarosegel electrophoresis, isolated, purified, cloned into the TOPO® cloningvector (Invitrogen), and partially sequenced to confirm its identity.

Targeted Disruption of the Yarrowia lipolytica DGAT1 Gene

Targeted disruption of the putative DGAT1 gene in Y. lipolytica ATCC#90812 was carried out by homologous recombination-mediated replacementof the endogenous DGAT1 gene with a targeting cassette (using themethodology described in Example 2). Specifically, the 1.6 kB isolatedYI DGAT1 ORF (SEQ ID NO:13) was used as a PCR template molecule toconstruct a YI DGAT1 targeting cassette consisting of: 5′ homologous YIDGAT1 sequence, the Yarrowia Leucine 2 (Leu2) gene, and 3′ homologous YIDGAT1 sequence. For this, each portion of the targeting cassette wasfirst individually amplified, using the primers set forth below:

-   -   Upper primer P214 and lower primer P215 (SEQ ID NOs:94 and 95,        respectively), for amplification of the 5′ homologous DGAT1        sequence;    -   Upper primer P216 and lower primer P217 (SEQ ID NOs:96 and 97,        respectively), for amplification of the 3′ homologous DGAT1        sequence; and,    -   Upper primer P218 and lower primer P219 (SEQ ID NOs:98 and 99,        respectively), for amplification of the Leu2 gene (GenBank        Accession No. AAA35244).        The PCRs were performed using Pfu Ultra polymerase (Stratagene,        Catalog #600630), as described in the General Methods, and        purified. The three correct-sized, purified fragments were mixed        together as template molecules for a second PCR reaction using        PCR primers P214 and P219 (SEQ ID NOs:94 and 99) to obtain the        YI DGAT1 disruption cassette.

The targeting cassette was gel purified and used to transform mid-logphase wildtype Y. lipolytica (ATCC #90812). Transformation was performedas described in the General Methods.

Transformants were plated onto Bio101 DOB/CSM-Leu selection plates andmaintained at 30° C. for 2 to 3 days. Several leucine prototrophs werescreened by PCR to confirm the targeted DGAT1 disruption. Specifically,one set of PCR primers (P226 and P227 [SEQ ID NOs:100 and 101,respectively]) was designed to amplify a junction between the disruptioncassette and native target gene. Another set of PCR primers (P214 andP217 [SEQ ID NOs:94 and 97, respectively]) was designed to detect thenative gene.

All of the leucine prototroph colonies were positive for the junctionfragment and negative for the native fragment. Thus, targetedintegration was confirmed in these strains. Disruption of the gene wasfurther confirmed by GC analysis of total lipids of one of the disruptedstrains, designated as “S-D1” (see Example 9).

In a similar manner, the DGAT1 targeting cassette was used to disruptthe DGAT1 gene in strains containing single disruptions in either PDAT(“S-P” from Example 3), DGAT2 (“S-D2” from Example 2), or doubledisruptions in PDAT and DGAT2 (“S-D2-P” from Example 4). This resultedin the creation of strains with double knockouts in DGAT1 and PDAT(“S-D1-P”), in DGAT2 and DGAT1 (“S-D2-D1”) and triple knockouts inDGAT2, DGAT1 and PDAT (“S-D2-D1-P”).

Example 8 Cloning of Yarrowia lipolytica Acyl-CoA:Sterol-Acyltransferase(ARE2) Gene and Disruption of the Endogenous ARE2 Gene

The present Example describes the use of degenerate PCR primers toisolate the full-length coding sequence of the Yarrowia lipolytica ARE2(encoded by ORF YALI-CDS2141.1 (Example 6)) and the use of the sequenceto disrupt the native gene in Y. lipolytica.

Cloning of a Putative ARE2 Sequence From Yarrowia lipolytica By PCRUsing Degenerate PCR Primers

The full length YI ARE2 ORF was cloned by PCR using degenerate PCRprimers P205 and P208 (SEQ ID NOs:102 and 103, respectively) and Y.lipolytica ATCC #76982 (from Example 2) genomic DNA as template. Thedegenerate primers were required, since the nucleotide sequence encodingYI ARE2 was not known. The PCR was performed using the protocoldescribed in Example 7. A PCR product of the expected size was detected.

Targeted Disruption of the Yarrowia lipolytica ARE2 Gene (Prophetic)

Targeted disruption of the ARE2 gene in Y. lipolytica ATCC #90812 willbe carried out by homologous recombination-mediated replacement of theendogenous ARE2 gene with a targeting cassette (as described in Example7). Specifically, the ˜1.6 kB isolated ORF encoding the putative YI ARE2protein (SEQ ID NO:15) will be used as a PCR template molecule toconstruct a YI ARE2 targeting cassette consisting of: 5′ homologous YIARE2 sequence, the Yarrowia Leucine 2 (Leu2) gene, and 3′ homologous YIARE2 sequence. For this, each portion of the targeting cassette will befirst individually amplified, as described in Example 7, using theprimers set forth below:

-   -   Upper primer P220 and lower primer P221 (SEQ ID NOs:104 and 105,        respectively), for amplification of the 5′ homologous ARE2        sequence;    -   Upper primer P222 and lower primer P223 (SEQ ID NOs:106 and 107,        respectively), for amplification of the 3′ homologous ARE2        sequence; and,    -   Upper primer P224 and lower primer P225 (SEQ ID NOs:108 and 109,        respectively), for amplification of the Leu2 gene.

Following purification, each of the correct-sized fragments will bemixed and utilized as template molecules in a PCR reaction using primersP220 and P223 to obtain the targeting cassette. Following gelpurification of the product, the targeting cassette will be used totransform mid-log phase wildtype and mutant Y. lipolytica (ATCC #90812)strains containing single disruptions in PDAT (“S-P” from Example 3),DGAT2 (“S-D2” from Example 2), DGAT1 (“S-D1” from Example 7), doubledisruptions in PDAT and DGAT2 (“S-D2-P” from Example 4), doubledisruptions in PDAT and DGAT1 (“S-D1-P” from Example 7), doubledisruptions in DGAT1 and DGAT2 (“S-D1-D2” from Example 7) or tripledisruptions in PDAT, DGAT2 and DGAT1 (“S-D1-D2-P” from Example 7).Transformation will be performed as described in the General Methods.

Transformants will be plated onto Bio101 DOB/CSM-Leu selection platesand maintained at 30° C. for 2 to 3 days. Several leucine prototrophswill be screened by PCR to confirm the targeted ARE2 disruption, usingthe methodology described in Example 7.

Example 9 Determination of TAG Content In Mutant and Wildtype Yarrowialipolytica Strains (ATCC #90812)

The present Example describes a comparison of TAG content in wildtypeand mutant Y. lipolytica ATCC #90812 containing: (1) single disruptionsin PDAT, DGAT2 and DGAT1; (2) double disruptions in PDAT and DGAT2,DGAT1 and PDAT, and DGAT1 and DGAT2; and (3) triple disruptions in PDAT,DGAT2 and DGAT1.

Specifically, single colonies of wildtype and mutant Y. lipolyticacontaining single disruptions in PDAT (“S-P”, from Example 3), DGAT2(“S-D2”, from Example 2), DGAT1 (“S-D1”, from Example 7), doubledisruptions in PDAT and DGAT2 (“S-D2-P”, from Example 4), DGAT1 and PDAT(“S-D1-P”, from Example 7), DGAT1 and DGAT2 (“S-D1-D2”, from Example 7),and triple disruptions (“S-D1-D2-P”, from Example 7) were separatelygrown using conditions that induce oleaginy. One loopful of cells fromeach culture was each individually inoculated into 3 mL YPD medium andgrown overnight on a shaker (300 rpm) at 30° C. The cells were harvestedand washed once in 0.9% NaCl and resuspended in 50 mL of HGM. Cells werethen grown on a shaker for 48 hrs. Cells were washed in water and thecell pellet was lyophilized. Twenty (20) mg of dry cell weight was usedfor total fatty acid by GC analysis and the oil fraction following TLC(infra) and GC analysis.

Thin Layer Chromatography (TLC)

The methodology used for TLC is described below in the following fivesteps: (1) The internal standard of 15:0 fatty acid (10 μl of 10 mg/mL)was added to 2 to 3 mg dry cell mass, followed by extraction of thetotal lipid using a methanol/chloroform method. (2) Extracted lipid (50μl) was blotted across a light pencil line drawn approximately 1 inchfrom the bottom of a 5×20 cm silica gel 60 plate, using 25-50 μlmicropipettes. (3) The TLC plate was then dried under N₂ and wasinserted into a tank containing about ˜100 mL 80:20:1 hexane:ethylether:acetic acid solvent. (4) After separation of bands, a vapor ofiodine was blown over one side of the plate to identify the bands. Thispermitted samples on the other side of the plate to be scraped using arazor blade for further analysis. (5) Basic transesterification of thescraped samples and GC analysis was performed, as described in theGeneral Methods.

Results from GC Analysis

GC results are shown below in Table 9. Cultures are described as the “S”strain (wildtype), “S-P” (PDAT knockout), “S-D1” (DGAT1 knockout),“S-D2” (DGAT2 knockout), “S-D1-D2” (DGAT1 and DGAT2 knockout), “S-P-D1”(PDAT and DGAT1 knockout), “S-P-D2” (PDAT and DGAT2 knockout) and“S-P-D1-D2” (PDAT, DGAT1 and DGAT2 knockout). Abbreviations utilizedare: “WT”=wildtype; “FAs”=fatty acids; “dcw”=dry cell weight; and, “FAs% dcw, % WT”=FAs % dcw relative to the % in wildtype, wherein the “S”strain is wildtype. TABLE 9 Lipid Content In Yarrowia ATCC #90812Strains With Single, Double Or Triple Disruptions In PDAT, DGAT2 AndDGAT1 Total Fatty Acids TAG Fraction FAs % FAs % Residual dcw, FAs, FAs% dcw, FAs, FAs % dcw, Strain DAG AT mg μg dcw % WT μg dcw % WT S D1,D2, P 32.0 797 15.9 100 697 13.9 100 S-D1 D2, P 78.8 723 13.6 86 61711.6 83 S-D2 D1, P 37.5 329 6.4 40 227 4.4 32 S-P D1, D2 28.8 318 6.0 38212 4.0 29 S-D1-D2 P 64.6 219 4.1 26 113 2.1 15 S-D1-P D2 76.2 778 13.484 662 11.4 82 S-D2-P D1 31.2 228 4.3 27 122 2.3 17 S-D1-D2-P None 52.2139 2.4 15 25 0.4 3The results in Table 9 provide evidence that DGAT1 is a DAG AT, sinceits disruption resulted in lower oil content compared to the wild typestrain. The results shown above also indicate the relative contributionof the three DAG ATs to oil biosynthesis. DGAT2 contributes the most,while PDAT and DGAT1 contribute equally but less than DGAT2. Theresidual oil content ca. 3% in the triple knockout strain may be thecontribution of the YI ARE2 (see Example 8).

Example 10 Generation of EPA-Producing Yarrowia lipolytica ATCC #20362Strain MU and Strain Y2067U

The present Example describes the construction of strain MU and strainY2067U, each derived from Yarrowia lipolytica ATCC #20362, wherein eachstrain was capable of producing significant concentrations of EPArelative to the total lipids (FIG. 5). The affect of variousacyltransferase knockouts and acyltransferase gene overexpression wasexamined in these EPA producing strains based on analysis of TAG contentand/or fatty acid composition, as described in Examples 11, 12 and 15(infra).

The development of strain MU herein required the construction of strainM4 (producing 8% DGLA), strain Y2034 (producing 10% ARA), strain E(producing 10% EPA), strain EU (producing 10% EPA) and strain M26(producing 14%). The development of strain Y2067U first required thecreation of a derivative of strain EU, designated as strain Y2067(producing 15% EPA).

Construction of Strain M4 Producing 8% DGLA

Construct pKUNF12T6E (FIG. 6A; SEQ ID NO: 10) was generated to integratefour chimeric genes (comprising a Δ12 desaturase, a Δ6 desaturase and 2elongases) into the Ura3 loci of wild type Yarrowia strain ATCC #20362,to thereby enable production of DGLA. The pKUNF12T6E plasmid containedthe following components: TABLE 10 Description of Plasmid pKUNF12T6E(SEQ ID NO: 110) RE Sites And Nucleotides Within SEQ ID Description OfFragment And NO: 110 Chimeric Gene Components AscI/BsiWI 784 bp 5′ partof Yarrowia Ura3 gene (Gen Bank (9420-8629) Accession No. AJ306421)SphI/PacI 516 bp 3′ part of Yarrowia Ura3 gene (GenBank (12128-1)Accession No. AJ306421) SwaI/BsiWI FBAIN::EL1S: Pex20, comprising:(6380-8629) FBAIN: FBAIN promoter (SEQ ID NO: 111; see also U.S. PatentApplication No. 60/519971) EL1S: codon-optimized elongase 1 gene (SEQ IDNO: 112), derived from Mortierella alpina (GenBank Accession No.AX464731) Pex20: Pex20 terminator sequence from Yarrowia Pex20 gene(GenBank Accession No. AF054613) BgIII/SwaI TEF::Δ6S::Lip1, comprising:(4221-6380) TEF: TEF promoter (GenBank Accession No. AF054508) Δ6S:codon-optimized Δ6 desaturase gene (SEQ ID NO: 114), derived fromMortierella alpina (GenBank Accession No. AF465281) Lip1: Lip1terminator sequence from Yarrowia Lip1 gene (GenBank Accession No.Z50020) PmeI/ClaI FBA::F.Δ12::Lip2, comprising: (4207-1459) FBA: FBApromoter (SEQ ID NO: 116; see also U.S. Patent Application No.60/519971) F.Δ12: Fusarium moniliforme Δ12 desaturase gene (SEQ ID NO:117) Lip2: Lip2 terminator sequence from Yarrowia Lip2 gene (GenBankAccession No. AJ012632) ClaI/PacI TEF::EL2S::XPR, comprising: (1459-1)TEF: TEF promoter (GenBank Accession No. AF054508) EL2S: codon-optimizedelongase gene (SEQ ID NO: 119), derived from Thraustochytrium aureum(U.S. 6,677,145) XPR: XPR terminator sequence of Yarrowia Xpr gene(GenBank Accession No. M17741)

The pKUNF12T6E plasmid was digested with AscI/SphI, and then used fortransformation of wild type Y. lipolytica ATCC #20362 according to theGeneral Methods. The transformant cells were plated onto FOA selectionmedia plates and maintained at 30° C. for 2 to 3 days. The FOA resistantcolonies were picked and streaked onto MM and MMU selection plates. Thecolonies that could grow on MMU plates but not on MM plates wereselected as Ura− strains. Single colonies of Ura− strains were theninoculated into liquid MMU at 30° C. and shaken at 250 rpm/min for 2days.

The cells were collected by centrifugation, lipids were extracted, andfatty acid methyl esters were prepared by trans-esterification, andsubsequently analyzed with a Hewlett-Packard 6890 GC.

GC analyses showed the presence of DGLA in the transformants containingthe 4 chimeric genes of pKUNF12T6E (FIG. 6A), but not in the wild typeYarrowia control strain. Most of the selected 32 Ura⁻ strains producedabout 6% DGLA of total lipids. There were 2 strains (i.e., strains M4and 13-8) that produced about 8% DGLA of total lipids.

Construction of Strain Y2034 Producing About 10% ARA

Constructs pDMW232 (FIG. 6B; SEQ ID NO:121) was generated to integratetwo Δ5 chimeric genes into the Leu2 gene of Yarrowia strain M4. Theplasmid pDMW232 contained the following components: TABLE 11 Descriptionof Plasmid pDMW232 (SEQ ID NO: 121) RE Sites And Nucleotides Within SEQID Description Of Fragment And NO: 121 Chimeric Gene ComponentsAscI/BsiWI 788 bp 5′ part of Yarrowia Leu2 gene (GenBank (5550-4755)Accession No. AF260230) SphI/PacI 703 bp 3′ part of Yarrowia Leu2 gene(GenBank (8258-8967) Accession No. AF260230) SwaI/BsiWIFBAIN::MAΔ5::Pex20, comprising: (2114-4755) FBAIN: FBAIN Promoter (SEQID NO: 111; see also U.S. Patent Application No. 60/519971) MAΔ5:Mortierella alpina Δ5 desaturase gene (SEQ ID NO: 122) (GenBankAccession No. AF067654) Pex20: Pex20 terminator sequence of YarrowiaPex20 gene (GenBank Accession No. AF054613) SwaI/ClaI TEF::MAΔ5::Lip1,comprising: (2114-17) TEF: TEF promoter (GenBank Accession No. AF054508)MAΔ5: as described for FBAIN::MAΔ5::Pex20 (supra) Lip1: Lip1 terminatorsequence of Yarrowia Lip1 gene (GenBank Accession No. Z50020) PmeI/ClaIYarrowia Ura3 gene (5550-4755) (GenBank Accession No. AJ306421)

Plasmid pDMW232 was digested with AscI/SphI, and then used to transformstrain M4 according to the General Methods. Following transformation,the cells were plated onto MMLe plates and maintained at 30° C. for 2 to3 days. The individual colonies grown on MMLe plates from eachtransformation were picked and streaked onto MM and MMLe plates. Thosecolonies that could grow on MMLe plates but not on MM plates wereselected as Leu2⁻ strains. Single colonies of Leu2⁻ strains were theninoculated into liquid MMLe media at 30° C. and shaken at 250 rpm/minfor 2 days. The cells were collected by centrifugation, lipids wereextracted, and fatty acid methyl esters were prepared bytrans-esterification, and subsequently analyzed with a Hewlett-Packard6890 GC.

GC analyses showed the presence of ARA in pDMW232 transformants, but notin the parental M4 strain. Specifically, among the 48 selected Leu2⁻transformants with pDMW232, there were 34 strains that produced lessthan 5% ARA, 11 strains that produced 6-8% ARA, and 3 strains thatproduced about 10% ARA of total lipids in the engineered Yarrowia. Oneof the strains that produced 10% ARA was named “Y2034”.

Construction of Strain E, Producing About 10% EPA

Construct pZP3L37 (FIG. 6C; SEQ ID NO:124) was created to integratethree synthetic Δ17 desaturase chimeric genes into the acyl-CoA oxidase3 (i.e., POX3) gene of the Y2034 strain. The plasmid pZP3L37 containedthe following components: TABLE 12 Description of Plasmid pZP3L37 (SEQID NO: 124) RE Sites And Nucleotides Within SEQ ID Description OfFragment And NO: 124 Chimeric Gene Components AscI/BsiWI 763 bp 5′ partof Yarrowia Pox3 gene (GenBank (6813-6043) Accession No. AJ001301)SphI/PacI 818 bp 3′ part of Yarrowia Pox3 gene (GenBank (9521-10345)Accession No. AJ001301) ClaI/BsiWI TEF::Δ17S::Pex20, comprising:(4233-6043) TEF: TEF promoter (GenBank Accession No. AF054508) Δ17S:codon-optimized Δ17 desaturase gene (SEQ ID NO: 125), derived from S.diclina (US 2003/0196217 A1) Pex20: Pex20 terminator sequence ofYarrowia Pex20 gene (GenBank Accession No. AF054613) ClaI/PmeIFBAIN::Δ17S::Lip2, comprising: (4233-1811) FBAIN: FBAIN promoter (SEQ IDNO: 111; see also U.S. Patent Application No. 60/519971) Δ17S: SEQ IDNO: 125 (supra) Lip2: Lip2 terminator sequence of Yarrowia Lip2 gene(GenBank Accession No. AJ012632) PmeI/SwaI Yarrowia Leu2 gene (GenBankAccession (1811-1) No. AF260230) PacI/SwaI FBAINm::Δ17S::Pex16,comprising: (10345-1) FBAINm: FBAINm promoter (SEQ ID NO: 127; see alsoU.S. Patent Application No. 60/519971) Δ17S: SEQ ID NO: 125 (supra)Pex16: Pex16 terminator sequence of Yarrowia Pex16 gene (GenBankAccession No. U75433)

Plasmid pZP3L37 was digested with AscI/SphI, and then used to transformstrain Y2034 according to the General Methods. Following transformation,the cells were plated onto MM plates and maintained at 30° C. for 2 to 3days. A total of 48 transformants grown on the MM plates were picked andre-streaked onto fresh MM plates. Once grown, these strains wereindividually inoculated into liquid MM at 30° C. and shaken at 250rpm/min for 2 days. The cells were collected by centrifugation, lipidswere extracted, and fatty acid methyl esters were prepared bytrans-esterification, and subsequently analyzed with a Hewlett-Packard6890 GC.

GC analyses showed the presence of EPA in most of the transformants withpZP3L37, but not in the parental strain (i.e., Y2034). Among the 48selected transformants with pZP3L37, there were 18 strains that producedless than 2% EPA, 14 strains that produced 2-3% EPA, and 1 strain thatproduced about 7% EPA of total lipids in the engineered Yarrowia.

The strain that produced 7% EPA was further analyzed after culturing thestrain as follows (“two-stage growth conditions”). First, cells weregrown in triplicate in liquid MM at 30° C. with shaking at 250 rpm/minfor 48 hrs. The cells were collected by centrifugation and the liquidsupernatant was extracted. The pelleted cells were resuspended in HGMand grown for 72 hrs at 30° C. with shaking at 250 rpm/min. The cellswere again collected by centrifugation and the liquid supernatant wasextracted.

GC analyses showed that the engineered strain produced about 10% EPA oftotal lipids after the two-stage growth. The strain was designated asthe “E” strain.

Construction of Strain EU Producing about 10% EPA

Strain EU (Ura⁻) was created by identifying mutant cells of strain Ethat were 5-FOA resistant. Specifically, one loop of Yarrowia E straincells were inoculated into 3 mL YPD medium and grown at 30° C. withshaking at 250 rpm for 24 hrs. The culture with diluted with YPD to anOD₆₀₀ of 0.4 and then incubated for an additional 4 hrs. The culture wasplated (100 μl/plate) onto FOA selection plates and maintained at 30° C.for 2 to 3 days. A total of 16 FOA resistant colonies were picked andstreaked onto MM and FOA selection plates. From these, 10 colonies grewon FOA selection plates but not on MM plates and were selected aspotential Ura⁻ strains.

One of these strains was used as host for transformation with pY37/F15,comprising a chimeric GPD::Fusarium moniliforme Δ15::XPR2 gene and aUra3 gene as a selection marker (FIG. 6D; SEQ ID NO:128). After threedays of selection on MM plates, hundreds of colonies had grown on theplates and there was no colony growth of the transformation control thatcarried no plasmid. This 5-FOA resistant strain was designated as strain“EU”.

Single colonies of the EU strain were then inoculated into liquid MMUadditionally containing 0.1 g/L uridine and cultured for 2 days at 30°C. with shaking at 250 rpm/min. The cells were collected bycentrifugation, lipids were extracted, and fatty acid methyl esters wereprepared by trans-esterification and subsequently analyzed with aHewlett-Packard 6890 GC. GC analyses showed that the EU strain producedabout 10% EPA of total lipids.

Construction of Strain M26. Producing 14% EPA

Construct pZKO2UM26E (FIG. 6E, SEQ ID NO:129) was used to integrate acluster of three chimeric genes (comprising an elongase, a Δ6 desaturaseand a Δ12 desaturase) and a Ura3 gene into the Yarrowia Δ12 desaturasegene site of strain EU. Plasmid pKO2UM26E contained the followingcomponents: TABLE 13 Description of Plasmid pKO2UM26E (SEQ ID NO: 129)RE Sites And Nucleotides Within SEQ ID Description Of Fragment And NO:129 Chimeric Gene Components HindIII/AscI 728 bp 5′ part of Yarrowia Δ12desaturase gene (SEQ ID (1-728) NO: 130) SphI/EcoRI 556 bp 3′ part ofYarrowia Δ12 desaturase gene (SEQ ID 3436-3992) NO: 130) BsiWI/HindIIIGPAT::EL1S::XPR, comprising: (11095-1) GPAT: GPAT promoter (SEQ ID NO:132; see also U.S. Patent Application No. 60/610060) EL1S:codon-optimized elongase 1 gene (SEQ ID NO: 112), derived fromMortierella alpina (GenBank Accession No. AX464731) XPR: terminatorsequence of Yarrowia Xpr2 gene (GenBank Accession No. M17741)BgIII/BsiWI FBAIN::M.Δ12.Pex20, comprising: (8578-11095) FBAIN: FBAINpromoter (SEQ ID NO: 111; see also U.S. Patent Application No.60/519971) M.Δ12: Mortieralla isabellina Δ12 desaturase gene (GenBankAccession No. AF417245; SEQ ID NO: 133) Pex20: Pex20 terminator sequenceof Yarrowia Pex20 gene (GenBank Accession No. AF054613) SalI/PacIYarrowia Ura3 gene (GenBank Accession (6704-8202) No. AJ306421)EcoRI/SalI FBAIN::M.Δ6B::Pex20, comprising: (3992-6704) FBAIN: FBAINpromoter (SEQ ID NO: 111; see also U.S. Patent Application No.60/519971) M.Δ6B: Mortieralla alpina Δ6 desaturase gene “B” (GenBankAccession No. AB070555; SEQ ID NO: 135) Pex20: Pex20 terminator sequenceof Yarrowia Pex20 gene (GenBank Accession No. AF054613)

The plasmid pKO2UM26E was digested with SphI/AscI, and then used totransform EU strain according to the General Methods. Followingtransformation, cells were plated onto MM plates and maintained at 30°C. for 2 to 3 days.

A total of 48 transformants grown on MM plates were picked andre-streaked onto fresh MM plates. Once grown, these strains wereindividually inoculated into liquid MM at 30° C. and grown with shakingat 250 rpm/min for 1 day. The cells were collected by centrifugation,lipids were extracted, and fatty acid methyl esters were prepared bytrans-esterification, and subsequently analyzed with a Hewlett-Packard6890 GC.

GC analyses showed that EPA was produced in almost all transformantswith pKO2UM26E after one-day growth in MM media. Among the 48 selectedtransformants, 5 strains produced less than 4% EPA, 23 strains produced4-5.9% EPA, 9 strains produced 6-6.9% EPA and 11 strains produced 7-8.2%EPA of total lipids in the engineered Yarrowia. The strain that produced8.2% EPA was selected for further analysis using a two-stage growthprocedure (i.e., 48 hrs MM+96 hrs in HGM). GC analyses showed that theengineered strain produced about 14% EPA of total lipids. The strain wasdesignated as strain “M26”. The final genotype of the M26 strain withrespect to wildtype Yarrowia lipolytica ATCC #20362 was as follows:Pox3-, Y.Δ12-, FBA::F.Δ12::Lip2, FBAIN::MΔ12::Pex20, TEF::Δ6S::Lip 1,FBAIN::Δ6B::Pex20, FBAIN::E1S::Pex20; GPAT::E1S:: Xpr, TEF::E2S::Xpr;FBAIN::MAΔ5::Pex20, TEF::MAΔ5::Lip 1, FBAIN::Δ17S::Lip2,FBAINm::Δ17S::Pex16 and TEF::Δ17S::Pex20.

Construction of Strain MU, Producing EPA

Strain MU was a Ura auxotroph of strain M26. This strain was made bytransforming strain M26 with 5 μg of plasmid PZKUM (FIG. 7A; SEQ IDNO:137) that had been digested with PacI and HincII. Transformation wasperformed using the Frozen-EZ Yeast Transformation kit (Zymo ResearchCorporation, Orange, Calif.) and transformants were selected by plating100 μl of the transformed cell mix on an agar plate with the followingmedium: 6.7 g/L yeast Nitrogen Base (DIFCO Laboratories, Detroit,Mich.), 20 g/L dextrose, 50 mg/L uracil and 800 mg/L FOA. After 7 days,small colonies appeared that were plated on MM and MMU agar plates. Allwere Ura auxotrophs. One of the strains was designated “MU”.

Construction of Strain Y2067 Producing About 15% EPA

Plasmid pKO2UF2PE (FIG. 7B; SEQ ID NO:138) was created to integrate acluster containing two chimeric genes (comprising a heterologous Δ12desaturase and an elongase) and a Ura3 gene into the native Yarrowia Δ12desaturase gene of strain EU (supra). Plasmid pKO2UF2PE contained thefollowing components: TABLE 14 Description of Plasmid pKO2UF2PE (SEQ IDNO: 138) RE Sites And Nucleotides Within SEQ ID Description Of FragmentAnd NO: 138 Chimeric Gene Components AscI/BsiWI 730 bp 5′ part ofYarrowia Δ12 desaturase gene (SEQ ID (3382-2645) NO: 130) SphI/EcoRI 556bp 3′ part of Yarrowia Δ12 desaturase gene (SEQ ID (6090-6646) NO: 130)SwaI/BsiWI/ FBAINm::F.Δ12DS::Pex20, comprising: (1-2645) FBAINm: FBAINmpromoter (SEQ ID NO: 127; see also U.S. Patent Application No.60/519971) F.Δ12: Fusarium moniliforme Δ12 desaturase gene (SEQ ID NO:117) Pex20: Pex20 terminator sequence of Yarrowia Pex20 gene (GenBankAccession No. AF054613) SwaI/PmeI GPAT::EL1S::OCT, comprising: (1-8525)GPAT: GPAT promoter (SEQ ID NO: 132; see also U.S. Patent ApplicationNo. 60/610060) EL1S: codon-optimized elongase 1 gene (SEQ ID NO: 112),derived from Mortierella alpina (GenBank Accession No. AX464731) OCT:OCT terminator sequence of Yarrowia OCT gene (GenBank Accession No.X69988) EcoRI/PacI Yarrowia Ura3 gene (GenBank Accession (6646-8163) No.AJ306421)

Plasmid pKO2UF2PE was digested with AscI/SphI and then used to transformstrain EU according to the General Methods (although strain EU wasstreaked onto a YPD plate and grown for approximately 36 hr prior tosuspension in transformation buffer [versus 18 hrs]). Followingtransformation, cells were plated onto MM plates and maintained at 30°C. for 2 to 3 days. A total of 72 transformants grown on MM plates werepicked and re-streaked separately onto fresh MM plates. Once grown,these strains were individually inoculated into liquid MM at 30° C. andshaken at 250 rpm/min for 2 days. The cells were collected bycentrifugation, lipids were extracted, and fatty acid methyl esters wereprepared by trans-esterification, and subsequently analyzed with aHewlett-Packard 6890 GC.

GC analyses showed the presence of EPA in almost all of thetransformants with pKO2UF2PE. More specifically, among the 72 selectedtransformants, there were 17 strains that produced 8-9.9% EPA, 27strains that produced 10-10.9% EPA, 16 strains that produced 11-11.9%EPA, and 7 strains that produced 12-12.7% EPA of total lipids in theengineered Yarrowia. The strain that produced 12.7% EPA was furtheranalyzed by using two-stage growth conditions. GC analyses showed thatthe engineered strain produced about 15% EPA of total lipids after thetwo-stage growth. The strain was designated as strain “Y2067”.

Construction of Strain Y2067U Producing about 14% EPA with Ura-Phenotype

In order to disrupt the Ura3 gene in Y2067 strain, construct pZKUT16(FIG. 7C; SEQ ID NO:139) was created to integrate a TEF::rELO2S::Pex20chimeric gene into the Ura3 gene of strain Y2067. rELO2S is acodon-optimized rELO gene encoding a rat hepatic enzyme that elongates16:0 to 18:0. The plasmid pZKUT16 contained the following components:TABLE 15 Description of Plasmid pZKUT16 (SEQ ID NO: 139) RE Sites AndNucleotides Within SEQ ID NO: 139 Description Of Fragment And ChimericGene Components BsiWI/PacI 721 bp 5′ part of Yarrowia Ura3 gene (GenBankAccession (1-721) No. AJ306421) SalI/ClaI 724 bp 3′ part of YarrowiaUra3 gene (GenBank Accession (3565-4289) No. AJ306421) ClaI/BsiWITEF::rELO2S::Pex20, comprising: (4289-1) TEF: TEF Promoter (GenBankAccession No. AF054508) rELO2S: codon-optimized rELO2 elongase gene (SEQID NO: 140), derived from rat (GenBank Accession No. AB071986) Pex 20:Pex20 terminator sequence of Yarrowia Pex20 gene (GenBank Accession No.AF054613)

The plasmid pZKUT16 was digested with SalI/PacI, and then used totransform Y2067 strain according to the General Methods. Followingtransformation, cells were plated onto FOA selection plates andmaintained at 30° C. for 2 to 3 days.

A total of 24 transformants grown on FOA plates were picked andre-streaked onto MM plates and FOA plates, separately. The strains thatcould grow on FOA plates, but not on MM plates, were selected as Ura−strains. A total of 10 Ura− strains were individually inoculated intoliquid MMU media at 30° C. and grown with shaking at 250 rpm/min for 1day. The cells were collected by centrifugation, lipids were extracted,and fatty acid methyl esters were prepared by trans-esterification, andsubsequently analyzed with a Hewlett-Packard 6890 GC.

GC analyses showed the presence of 5 to 7% EPA in all of thetransformants with pZKUT16 after one day growth in MMU media. The strainthat produced 6.2% EPA was further analyzed using two-stage growthconditions (48 hrs MM+96 hrs in HGM). GC analyses showed that theengineered strain produced about 14% EPA of total lipids. The strain wasdesignated as strain “Y2067U”. The final genotype of this strain withrespect to wildtype Yarrowia lipolytica ATCC #20362 was as follows:Ura3-, Pox3-, Y.Δ12-, FBA::F.Δ12::Lip2, FBAINm::F. Δ12::Pex20,TEF::A6S::Lip1, FBAIN::E1S::Pex20; GPAT::EIS:: Oct, TEF::E2S::Xpr;FBAIN::MAΔ5::Pex20, TEF::MAA5::Lip 1, FBAIN::Δ17S::Lip2,FBAINm::Δ17S::Pex16, TEF::Δ17S::Pex20 and TEF::rELO2S::Pex20.

Example 11 Determination of TAG Content in Mutant and Wildtype Yarrowialipolytica Strain MU Engineered for EPA Biosynthesis

The present Example describes TAG content and fatty acid composition invarious acyltransferase knockout strains of strain MU (an engineeredstrain of Yarrowia lipolytica ATCC #20362, capable of producing greaterthan 10% EPA). More specifically, single disruptions in PDAT, DGAT2 andDGAT1 and double disruptions in PDAT and DGAT2 were created in strainMU. Lipid content and composition is compared in each of these strains,following growth in 4 different growth conditions.

More specifically, single disruptions in PDAT, DGAT2, DGAT1 were createdin strain MU (supra, Example 10), using the methodology described inExamples 2, 3, and 7 (with the exception that selection for the DGAT1disruption relied on the URA3 gene). This resulted in single knockoutstrains identified as “MU-D1” (disrupted in DGAT1), “MU-D2” (disruptedin DGAT2) and “MU-P” (disrupted in PDAT). Individual knockout strainswere confirmed by PCR. Additionally, the MU-D2 strain was disrupted forthe PDAT gene and the disruption confirmed by PCR. The resulting doubleknockout strain was designated “MU-D2-P”.

The MU-D1, MU-D2, MU-P and M-D2-P knockout strains were analyzed todetermine each knockout's effect on lipid content and composition, asdescribed below. Furthermore, the growth conditions promoting oleaginywere also explored to determine their effect on total lipid content.Thus, in total, four different experiments were conducted, identified as“Experiment A”, “Experiment B”, “Experiment C” and “Experiment E”.Specifically, three loops of cells from plates containing each strainabove were inoculated into MMU [3 mL for Experiments B and C; and 50 mLfor Experiments A and E] and grown in a shaker at 30° C. for 24 hrs (forExperiments A, B and C) or 48 hrs (for Experiment E). Cells wereharvested, washed once in HGM, resuspended in either HGM (50 mL forExperiments A and E; and 3 mL for Experiment B) or HGM with uracil(“HGMU”) (3 mL for Experiment C) and cultured as above for 4 days. Onealiquot (1 mL) was used for lipid analysis by GC as described accordingto the General Methods, while a second aliquot was used for determiningthe culture OD at 600 nm. The remaining culture in Experiments A and Ewas harvested, washed once in water and lyophilized for dry cell weight(dcw) determination. In contrast, the dcw in Experiments B and C weredetermined from their OD₆₀₀ using the equation showing theirrelationship. The fatty acid compositions of each of the differentstrains in Experiments A, B, C and E were also determined.

The results are shown in Table 16 below. Cultures are described as the“MU” strain (the parent EPA producing strain), “MU-P” (PDAT knockout),“MU-D1” (DGAT1 knockout), “MU-D2” (DGAT2 knockout) and “MU-D2-P” (DGAT2and PDAT knockouts). Abbreviations utilized are: “WT”=wildtype (i.e.,MU); “OD”=optical density; “dcw”=dry cell weight; “TFAs”=total fattyacids; and, “TFAs % dcw, % WT”=TFAs % dcw relative to the wild type(“MU”) strain. Fatty acids are identified as 16:0, 16:1, 18:0, 18:1(oleic acid), 18:2 (LA), GLA, DGLA, ARA, ETA and EPA; and thecomposition of each is presented as a % of the total fatty acids. TABLE16 Lipid And EPA Content In Yarrowia Strain MU With VariousAcyltransferase Disruptions 1^(st) Phase 2^(nd) Phase TFAs ResidualGrowth Growth dcw TFAs TFAs % % dcw, Expt Strain DAG AT ConditionCondition OD (mg) (μg) dcw % WT A MU D1, D2, P 1 day, 4 days, 4.0 91 37420.1 100 A MU-D2 D1, P 50 mL 50 mL 3.1 75 160 10.4 52 A MU-D1 D2, P MMUHGM 4.3 104 217 10.2 51 A MU-P D1, D2 4.4 100 238 11.7 58 B MU D1, D2, P1 day, 4 days, 5.9 118 581 24.1 100 B MU-D2 D1, P 3 mL 3 mL 4.6 102 24811.9 50 B MU-D1 D2, P MMU HGM 6.1 120 369 15.0 62 B MU-P D1, D2 6.4 124443 17.5 72 C MU D1, D2, P 1 day, 4 days, 6.8 129 522 19.9 100 C MU-D2D1, P 3 mL 3 mL 5.6 115 239 10.2 51 C MU-D1 D2, P MMU HGMU 6.9 129 39515.0 75 C MU-P D1, D2 7.1 131 448 16.8 84 E MU D1, D2, P 2 days, 4 days,4.6 89 314 17.3 100 E MU-D2 D1, P 50 mL 50 mL 2.8 62 109 8.5 49 E MU-PD2, P MM HGM 5.0 99 232 11.5 66 E MU-D2-P D1 4.2 98 98 4.9 28 % % % % %% % % % % Expt 16:0 16:1 18:0 18:1 18:2 GLA DGLA ARA ETA EPA A 17 10 218 10 22 7 1 3 9.7 A 16 12 0 8 9 23 7 0 8 17.4 A 15 10 2 11 10 22 7 0 717.4 A 16 9 2 11 7 24 7 1 6 17.5 B 17 9 3 18 10 22 8 1 3 9.1 B 16 10 0 710 24 7 1 7 17.8 B 18 9 3 14 11 20 7 1 5 12.0 B 15 8 3 16 10 25 6 1 411.9 C 16 10 2 13 11 21 10 1 4 12.6 C 17 9 1 6 11 21 8 1 7 18.9 C 15 9 212 12 20 10 1 5 13.5 C 17 8 3 14 11 20 10 1 4 11.3 E 16 12 2 18 9 22 7 14 11.2 E 14 12 1 6 8 25 6 0 7 20.0 E 16 10 2 14 7 24 7 1 5 15.8 E 18 100 7 12 20 5 0 6 22.5

The data showed that the lipid content within the transformed cellsvaried according to the growth conditions. Furthermore, the contributionof each acyltransferase on lipid content also varied. Specifically, inExperiments B, C and E, DGAT2 contributed more to oil biosynthesis thaneither PDAT or DGAT1. In contrast, as demonstrated in Experiment A, asingle knockout in DGAT2, DGAT1 and PDAT resulted in approximatelyequivalent losses in lipid content (i.e., 48%, 49% and 42% loss,respectively [see “TFAs % dcw, % WT”]).

Example 12 Sequencing of Yarrowia lipolytica DGAT1 and ORF ExpressionUnder the Control of a Yarrowia Promoter

The present Example describes the sequencing of YI DGAT1 and theover-expression of a chimeric gene comprising the Yarrowia lipolyticaTEF promoter, YI DGAT1, and Yarrowia lipolytica peroxin (Pex20)terminator (i.e., a TEF::YI DGAT1::Pex20 gene) in a wild type Yarrowiastrain.

Sequencing of the Y. lipolytica DGAT1

First, the ORF of Y. lipolytica DGAT1 was PCR-amplified using degenerateprimers P201 and P203 (SEQ ID NOs:92 and 93) and genomic DNA of Y.lipolytica ATCC #90812 as template (from Example 2). The PCR wasperformed using the Expand High Fidelity PCR System of Roche AppliedSciences (Indianapolis, Ind.), as described in the General Methods.

The expected 1.6 kB fragment was isolated by standard agarose gelelectrophoresis, purified, and cloned into pCR4-TOPO vector fromInvitrogen (Carlsbad, Calif.) to yield plasmid pYAP42-23. PlasmidpYAP42-23 was transformed into E. coli XL2; and, transformantscomprising pYAP42-23 were confirmed by plasmid miniprep analysis andrestriction enzyme digestions with either NotI or NcoI. The DNA insertin plasmid pYAP42-23 was sequenced according to the methodologydescribed in the General Methods using sequencing primers T7, T3, P239(SEQ ID NO:142) and P240 (SEQ ID NO:143), to obtain the completenucleotide sequence of the YI DGAT1 ORF.

The nucleotide sequence of the YI DGAT1 ORF is provided as SEQ ID NO:13;the translated product has the amino acid sequence provided in SEQ IDNO:14. The resultant sequence was compared to other known proteins,based on BLAST program analysis (Basic Local Alignment Search Tool;Altschul, S. F. et al., J. Mol. Biol. 215:403-410 (1993)). Inparticular, SEQ ID NO:13 was identical to the YI DGAT1 partial sequencethat was obtained in Example 7, except for the presence of 6 silentmutations in the region of the degenerate PCR primers. These mutationsincluded: an A-to-G mutation at position 6; an A-to-G mutation atposition 21; an A-to-G mutation at position 24; a T-to-C mutation atposition 1548; a C-to-T mutation at position 1552; and a T-to-C mutationat position 1557. Since these mutations resulted from the use ofdegenerate PCR primers, the deduced amino acid sequence of SEQ ID NO:13,i.e., SEQ ID NO:14 is identical to ORF YALI-CDS2141.1 (SEQ ID NO:12,corresponding to GenBank Accession No. NC_(—)006072,locus_tag=“YALI0F06578g”).

Construction of a Y. lipolytica DGAT1 Chimeric Gene

Plasmid pYAP23-42 was digested with NcoI and Not I for 1 hr and the 1.6kB fragment containing YI DGAT1 was isolated and inserted into NcoI- andNot I-digested pZP2I7+Ura (SEQ ID NO:144; FIG. 7DA), such that the ORFwas cloned under the control of the TEF promoter and the PEX20-3′terminator region in an integrating vector targeted into the YarrowiaPOX2 gene. Correct transformants were confirmed by miniprep analysis andthe resultant plasmid was designated as “pYDA1”.

Plasmids pZP217+Ura and pYDA1 were transformed into strain “MU-D2” of Y.lipolytica (supra, Example 11), according to the General Methods.Transformants were plated onto MM and single colonies were picked,purified, and analyzed to determine the effect of the overexpressedDGAT1 on lipid content. Specifically, lipid content was analyzed in thefollowing cultures: “MU” (“wildtype”), MU-D2 transformed withpZP217+Ura, and MU-D2 transformed with pYDA1 (clones #5, 6, 7 and 16).Several loops of cells from each of the strains above were inoculatedinto 50 mL MM and grown in a shaker at 30° C. for 48 hrs. Cells wereharvested, washed once in HGM, resuspended in 30 mL HGM medium, andgrown as above for another 4 days. After growth, 100 μL aliquots fromeach culture were used to determine absorbance at 600 nm (OD₆₀₀) and a 1mL aliquot was used for GC analysis. For this, the 1 mL sample washarvested, washed once in water, spun down, and the pellet used forlipid determination following direct transesterification by base methodand GC analysis (as described in the General Methods). The remainingculture was harvested, washed once in water and lyophilized to obtainthe dry cell weight.

The results are shown in the Table below. Cultures are described as the“MU” strain (“wildtype”) and “MU-D2” (DGAT2 knockout). Abbreviationsutilized are: “WT”=wildtype (i.e., strain MU-D2 having a DGAT2knockout); “OD”=optical density; “dcw”=dry cell weight; “TFAs”=totalfatty acids; and, “TFAs % dcw, % WT”=TFAs % dcw relative to the wildtype (“MU-D2”) strain. TABLE 17 Lipid Content In Yarrowia StrainsEngineered To Produce EPA And Overexpressing DGAT1 TFAs dcw, TFAs, TFAs% dcw, Strain OD mg μg % dcw % WT MU 2.1 33 195 17.0 MU-D2 + pZP2l7 +Ura 1.8 31 82 7.7 100 MU-D2 + pYDA1, clone #5 2.1 31 192 17.8 231MU-D2 + pYDA1, clone #6 1.3 22 338 21.3 278 MU-D2 + pYDA1, clone #7 4.472 146 5.9 76 MU-D2 + pYDA1, clone #16 2.2 34 195 16.9 220In general, the results showed that overexpression of DGAT1 was able tocompensate for the lack of DGAT2 activity in strain MU-D2, to result inlipid content approximately equal or greater than in strain MU (i.e.,MU-D2+pYDA1, clones #5, 6 and 16 have a lipid content (measured as TFAs% dcw) approximately equal to or greater than that of strain MU). Thislipid content in MU-D2+pYDA1, clones #5, 6 and 16 was greater thandouble that of the control strain, MU-D2+pZP217+Ura. These resultsprovide further confirmation that the Yarrowia DGAT1 encodes afunctional DAG AT involved in oil biosynthesis.

Transformant MU-D2+pYDA1, clone #7 did not show an increase in lipidcontent comparable to clones #5, 6 and 16. However, since thesechromosomal integrations are generally random, such variation is to beexpected.

Example 13 Construction and Sequencing of a Mortierella alpina cDNALibrary

The present Example describes the construction of a cDNA library ofMortierella alpina and subsequent sequencing of the library.

Synthesis of M. alpina cDNA

M. alpina cDNA was synthesized using the BD-Clontech Creator Smart® cDNAlibrary kit (Mississauga, ON, Canada), according to the manufacturer'sprotocol.

Specifically, M. alpina strain ATCC #16266 was grown in 60 mL YPD medium(2% Bacto-yeast extract, 3% Bactor-peptone, 2% glucose) for 3 days at23° C. Cells were pelleted by centrifugation at 3750 rpm in a BeckmanGH3.8 rotor for 10 min and resuspended in 6×0.6 mL Trizole reagent(Invitrogen). Resuspended cells were transferred to six 2 mL screw captubes each containing 0.6 mL of 0.5 mm glass beads. The cells werehomogenized at the HOMOGENIZE setting on a Biospec (Bartlesville, Okla.)mini bead beater for 2 min. The tubes were briefly spun to settle thebeads. Liquid was transferred to 4 fresh 1.5 mL microfuge tubes and 0.2mL chloroform/isoamyl alcohol (24:1) was added to each tube. The tubeswere shaken by hand for 1 min and let stand for 3 min. The tubes werethen spun at 14,000 rpm for 10 min at 4° C. The upper layer wastransferred to 4 new tubes. Isopropyl alcohol (0.5 mL) was added to eachtube. Tubes were incubated at room temperature for 15 min, followed bycentrifugation at 14,000 rpm and 4° C. for 10 min. The pellets werewashed with 1 mL each of 75% ethanol, made with RNase free water andair-dried. The total RNA sample was then redissolved in 500 μl of water,and the amount of RNA was measured by A260 nm using 1:50 diluted RNAsample. A total of 3.14 mg RNA was obtained.

This total RNA sample was further purified with the Qiagen RNeasy totalRNA Midi kit following the manufacturer's protocol. Thus, the total RNAsample was diluted to 2 mL and mixed with 8 mL of buffer RLT with 80 μlof β-mercaptoethanol and 5.6 mL 100% ethanol. The sample was dividedinto 4 portions and loaded onto 4 RNeasy midid columns. The columns werethen centrifuged for 5 min at 4500×g. To wash the columns, 2 mL ofbuffer RPE was loaded and the columns centrifuged for 2 min at 4500×g.The washing step was repeated once, except that the centrifugation timewas extended to 5 min. Total RNA was eluted by applying 250 μl of RNasefree water to each column, waiting for 1 min and centrifuging at 4500×gfor 3 min.

PolyA(+)RNA was then isolated from the above total RNA sample, followingPharmacia's kit protocol. Briefly, 2 oligo-dT-cellulose columns wereused. The columns were washed twice with 1 mL each of high salt buffer.The total RNA sample from the previous step was diluted to 2 mL totalvolume and adjusted to 10 mM Tris/HCl, pH 8.0, 1 mM EDTA. The sample washeated at 65° C. for 5 min, then placed on ice. Sample buffer (0.4 mL)was added and the sample was then loaded onto the two oligo-dT-cellulosecolumns under gravity feed. The columns were centrifuged at 350×g for 2min, washed 2× with 0.25 mL each of high salt buffer, each time followedby centrifugation at 350×g for 2 min. The columns were further washed 3times with low salt buffer, following the same centrifugation routine.Poly(A)+ RNA was eluted by washing the column 4 times with 0.25 mL eachof elution buffer preheated to 65° C., followed by the samecentrifugation procedure. The entire purification process was repeatedonce. Purified poly(A)+ RNA was obtained with a concentration of 30.4ng/μl.

cDNA was generated, using the LD-PCR method specified by BD-Clontech and0.1 μg of polyA(+) RNA sample. Specifically, for 1^(st) strand cDNAsynthesis, 3 μl of the poly(A)+ RNA sample was mixed with 1 μl of SMARTIV oligo nucleotide (SEQ ID NO:145) and 1 μl of CDSIII/3′ PCR primer(SEQ ID NO:146). The mixture was heated at 72° C. for 2 min and cooledon ice for 2 min. To the tube was added the following: 2 μl first strandbuffer, 1 μl 20 mM DTT, 1 μl 10 mM dNTP mix and 1 μl Powerscript reversetranscriptase. The mixture was incubated at 42° C. for 1 hr and cooledon ice.

The 1^(st) strand cDNA synthesis mixture was used as template for thePCR reaction. Specifically, the reaction mixture contained thefollowing: 2 μl of the 1^(st) strand cDNA mixture, 2 μl 5′-PCR primer(SEQ ID NO:147), 2 μl CDSIII/3′-PCR primer (SEQ ID NO:146),80 μl water,10 μl 10× Advantage 2 PCR buffer, 2 μl 50× dNTP mix and 2 μl 50×Advantage 2 polymerase mix. The thermocycler conditions were set for 95°C. for 20 sec, followed by 14 cycles of 95° C. for 5 sec and 68° C. for6 min on a GenAmp 9600 instrument. PCR product was quantitated byagarose gel electrophoresis and ethidium bromide staining.

Seventy-five μl of the above PCR products (cDNA) were mixed with 3 μl of20 μg/μl proteinase K supplied with the kit. The mixture was incubatedat 45° C. for 20 min, then 75 μl of water was added and the mixture wasextracted with 150 μl phenol:chloroform:isoamyl alcohol:mixture(25:24:1). The aqueous phase was further extracted with 150 μlchloroform:isoamyl alcohol (25:1). The aqueous phase was then mixed with15 μl of 3 M sodium acetate, 2 μl of 20 μg/μl glycogen and 400 μl of100% ethanol. The mixture was immediately centrifuged at roomtemperature for 20 min at 14000 rpm in a microfuge. The pellet waswashed once with 150 μl of 80% ethanol, air dried and dissolved in 79 μlof water.

Dissolved cDNA was subsequently digested with SfiI (79 μl of the cDNAwas mixed with 10 μl of 10× SfiI buffer, 10 μl of SfiI enzyme and 1 μlof 100× BSA and the mixture was incubated at 50° C. for 2 hrs). Xylenecyanol dye (2 μl of 1%) was added. The mixture was then fractionated onthe Chroma Spin-400 column provided with the kit, following themanufacturer's procedure exactly. Fractions collected from the columnwere analyzed by agarose gel electrophoresis. The first three fractionscontaining cDNA were pooled and cDNA precipitated with ethanol. Theprecipitated cDNA was redissolved in 7 μl of water, and ligated intokit-supplied pDNR-LIB.

Library Sequencing

The ligation products were used to transform E. coli XL-1 Blueelectroporation competent cells (Stratagene). An estimated total of2×10⁶ colonies was obtained. Sequencing of the cDNA library was carriedout by Agencourt Bioscience Corporation (Beverly, Mass.), using an M13forward primer (SEQ ID NO:148).

Example 14 Identification and Cloning of a Mortierella alpinaDiacylglycerol Acyltransferase (DGAT1) Gene

The present Example describes the identification of a putative M. alpinaDGAT1 within one of 9,984 cDNA sequences. Specifically, the Y.lipolytica DGAT1 protein sequence (Example 7, SEQ ID NO:14) was used asa query sequence against each of the M. alpina cDNA sequences usingBLAST program analysis (Basic Local Alignment Search Tool; Altschul, S.F. et al., J. Mol. Biol. 215:403-410 (1993)). One cDNA fragment boresignificant homology to the Y. lipolytica DGAT1 and thus was tentativelyidentified as the M. alpina DGAT1 (SEQ ID NO:175). Subsequent BLASTanalyses with SEQ ID NO:175 as the query against publically availablesequence databases confirmed the cDNA's significant degree of similaritywith the DGAT1s from several other species. Rapid amplification of cDNAends (RACE) technology and genome walking were then used to isolate theentire Mortierella alpina coding sequence thereof.

Isolation of Genomic DNA

Genomic DNA was isolated from Mortierella alpina (ATCC #16266) using aQiaPrep Spin Miniprep Kit (Qiagen, Catalog #627106). Cells grown on aYPD agar plate were scraped off and resuspended in 1.2 mL kit buffer P1.The resuspended cells were placed into two 2.0 mL screw cap tubes, eachcontaining 0.6 mL glass beads (0.5 mm diameter). The cells werehomogenized at the HOMOGENIZE setting on a Biospec (Bartlesville, Okla.)mini bead beater for 2 min. The tubes were then centrifuged at 14,000rpm in an Eppendorf microfuge for 2 min. The supernatant (0.75 mL) wastransferred to three 1.5 mL microfuge tubes. Equal volumes of kit bufferP2 were added to each tube. After mixing the tubes three times byinversion, 0.35 mL of buffer N3 was added to each tube. The contents wasmixed again by inverting the tubes 5 times. The mixture was centrifugedat 14,000 rpm in an Eppendorf microfuge for 5 min. The supernatant fromeach tube was transferred into a separate kit spin column. The columnswere centrifuged for 1 min at 14,000 rpm, washed once with buffer PE andcentrifuged at 14,000 rpm for 1 min again, followed by a finalcentrifugation at 14,000 rpm for 1 min. Buffer EB (50 μl) was added toeach column and let stand for 1 min. The genomic DNA was then eluted bycentrifugation at 14,000 rpm for 1 min.

Cloning of the 5′-End Region of the Putative DGAT1 Gene

A Clontech Universal GenomeWalker™ kit (Palo Alto, Calif.) was utilizedto obtain a piece of genomic DNA corresponding to the 5′-end region ofthe M. alpina DGAT1. Based on the partial DGAT1 gene sequence available(SEQ ID NO:149), the following primers were synthesized for use in thecloning: MARE2-N1 and MARE2-N2 (SEQ ID NOs:150 and 151).

Briefly, 2.5 μg each of M. alpina genomic DNA was digested with DraI,EcoRV, PvuII or StuI individually, the digested DNA samples werepurified using Qiagen Qiaquick PCR purification kits and eluted with 30μl each of kit buffer EB, and the purified samples were then ligatedwith Genome Walker adaptor (SEQ ID NOs:152 [top strand] and 153 [bottomstrand]), as shown below:5′-GTAATACGACTCACTATAGGGCACGCGTGGTCGACGGCCCGGGCTGG T-3′3′-H2N-CCCGACCA-5′Specifically, each ligation reaction mixture contained 1.9 μl of 25 μMGenome Walker adaptor, 1.6 μl 10× ligation buffer, 0.5 μl T4 DNA ligaseand 4 μl of one of the purified digested genomic DNA samples. Thereaction mixtures were incubated at 16° C. overnight. The reaction wasterminated by incubation at 70° C. for 5 min. Then, 72 μl of 10 mM TrisHCl, 1 mM EDTA, pH 7.4 buffer was added to each ligation reaction mix.

Four PCR reactions were then carried out using each of the ligationproducts as templates. Each PCR reaction mixture contained 1 μl ofligation mixture, 1 μl of 20 μM MARE2-N1 (SEQ ID NO:150), 2 μl of 10 μMkit primer AP1 (SEQ ID NO:154), 21 μl water and 25 μl ExTaq premix Taq2×PCR solution (TaKaRa Bio Inc., Otsu, Shiga, 520-2193, Japan). PCRamplifications were carried out as follows: 30 cycles of denaturation at94° C. for 30 sec, annealing at 55° C. for 30 sec, and elongation at 72°C. for 90 sec. A final elongation cycle at 72° C. for 7 min was carriedout, followed by reaction termination at 4° C.

Second PCR reactions were then carried out using 1 μl of 1:50 dilutedfirst PCR product as template, 1 μl of 20 μM MARE2-N2 (SEQ ID NO:151), 2μl of 10 uM kit primer AP2 (SEQ ID NO:155), 21 μl water and 25 μl ofExTaq premix Taq 2×PCR solution (TaKaRa). PCR reaction was carried outfor 30 cycles using the same conditions described above.

A ˜1.6 kb PCR product was observed when the DraI-digested andadaptor-ligated genomic DNA was used as template. This fragment waspurified using a Qiagen PCR purification kit, ligated into pCR2.1-TOPO,and sequenced. Analysis of the sequence (SEQ ID NO:156) showed that thisDNA fragment was the 5′-end extension of the DGAT1 cDNA fragment.

Cloning of the 3′-End Region of the Putative DGAT1 Gene

To clone the 3′-region of the putative DGAT1 gene by RACE, the followingprimers were synthesized: ARE-N-3-1 and ARE-N-3-2 (SEQ ID NOs:157 and158, respectively).

3′-end RACE was carried out using InVitrogen's 3′-end RACE kit,following the manufacturer's protocol. Briefly, 90 ng of M. alpinapolyA(+)RNA sample in 11 μl of water were mixed with 1 μl of 10 μMAdaptor primer (AP, SEQ ID NO:159) solution. The mixture was heated at70° C. for 10 min and cooled on ice for 2 min. Then, 2 μl each of 10×PCRbuffer, 25 mM MgCl₂ and 0.1 M DTT, and 1 μl of 10 mM dNTP mix wereadded. The reaction mixture was heated to 42° C. for 3 min and thenkit-supplied Superscript II reverse transcriptase (1 μl) was added. Thereaction was allowed to proceed for 50 min at 42° C. Afterward, thereaction mixture was heated to 70° C. for 15 min and cooled on ice for 2min. RNaseH (1 μl) from the kit was added and the entire mixture wasincubated at 37° C. for 20 min.

The reaction mixture (2 μl) was used directly as PCR template. The PCRreaction mixture contained 1 μl of 20 μM ARE-N-3-1 (SEQ ID NO:157), 2 μlof 10 uM kit primer UAP (SEQ ID NO:160), 25 μl of ExTaq premix Taq 2×PCRsolution (TaKaRa) and 20 μl of water. PCR amplification was performed aspreviously described above. Diluted PCR reaction mixture (1 μl of a 1:10dilution) was then used as template for a second round of PCR using thesame conditions, except with primer ARE-N3-2 (SEQ ID NO:158) replacingprimer AREN3-1.

A ca. 300 bp fragment was obtained from the PCR. After purification withQiagen's QiaQuick PCR purification kit, the fragment was cloned intopCR2.1-TOPO and sequenced. Sequence analysis verified that the sequenceencoded the 3′-end of the DGAT1 cDNA, including the polyA tail (SEQ IDNO:161).

Complete Assembly of the Nucleotide Sequence Encoding M. alpina's DGAT1

Assembly of the sequence of the 5′-region (SEQ ID NO:156), the originalcDNA fragment (SEQ ID NO:149) and the 3′-region (SEQ ID NO:161) yieldedthe entire M. alpina DGAT1 coding sequence (SEQ ID NO:17). The 5-regiongenomic sequence included an intron (nucleotide bases 449 to 845 withinSEQ ID NO:17).

Example 15

Expression of Mortierella alpina DGAT1 In Yarrowia lipolytica StrainY2067U Engineered To Produce Polyunsaturated Fatty Acids The presentExample describes the expression of the M. alpina DGAT1 in Y. lipolyticastrain Y2067U (supra, Example 10), and the effect of MDGAT1 expressionon the final concentration of EPA and other PUFAs produced.

The M. alpina DGAT1 ORF was cloned as follows. First, to aid the cloningof the cDNA, the sequence of the second codon of the DGAT1 was changedfrom ‘ACA’ to ‘GCA’, resulting in an amino acid change of threonine toalanine. This was accomplished by amplifying the complete coding regionof the M. alpina DGAT1 ORF with primers MACAT-F1 and MACAT-R (SEQ IDNOs:162 and 163, respectively). Specifically, the PCR reaction mixturecontained 1 μl each of a 20 μM solution of primers MACAT-F1 and MACAT-R,1 μl of M. alpina cDNA (supra, Example 13), 22 μl water and 25 μl ExTaqpremix 2× Taq PCR solution (TaKaRa Bio Inc., Otsu, Shiga, 520-2193,Japan). Amplification was carried out as follows: initial denaturationat 94° C. for 150 sec, followed by 30 cycles of denaturation at 94° C.for 30 sec, annealing at 55° C. for 30 sec and elongation at 72° C. for90 sec. A final elongation cycle at 72° C. for 10 min was carried out,followed by reaction termination at 4° C. A ˜1600 bp DNA fragment wasobtained from the PCR reaction. It was purified using Qiagen's PCRpurification kit according to the manufacturer's protocol.

The M. alpina DGAT1 ORF was to be inserted into Nco I- and NotI-digested plasmid pZUF17 (SEQ ID NO:164; FIG. 7E), such that the ORFwas cloned under the control of the FBAIN promoter (SEQ ID NO:111) andthe PEX20-3′ terminator region. However, since the DGAT1 ORF containedan internal NcoI site, it was necessary to perform two separaterestriction enzyme digestions for cloning. First, ˜2 μg of the purifiedPCR product was digested with BamHI and Nco 1. The reaction mixturecontained 20 U of each enzyme (Promega) and 6 μl of restriction buffer Din a total volume of 60 μl. The mixture was incubated for 2 hrs at 37°C. A 320 bp fragment was separated by agarose gel electrophoresis andpurified using a Qiagen Qiaex II gel purification kit. Separately, ˜2 μgof the purified PCR product was digested with BamHI and Not I usingidentical reaction conditions to those above, except Nco I was replacedby Not 1. A ˜1280 bp fragment was isolated and purified as above.Finally, 3 μg of pZUF17 was digested with Nco I and Not I and purifiedas described above, generating a ˜7 kB fragment.

The ˜7 kB Nco I/Not I pZUF17 fragment, the ˜320 bp Nco I/BamHI DGAT1fragment and the ˜1280 bp BamHI/Not I DGAT1 fragment were ligatedtogether in a three-way ligation incubated at room temperatureovernight. The ligation mixture contained 100 ng of the 7 kB fragmentand 200 ng each of the 320 bp and 1280 bp fragments, 2 μl ligase buffer,and 2 U T4 DNA ligase (Promega) in a total volume of 20 μl. The ligationproducts were used to transform E. coli Top10 chemical competent cells(Invitrogen) according to the manufacturer's protocol. Individualcolonies (12 total) from the transformation were used to inoculatecultures for miniprep analysis. Restriction mapping and sequencingshowed that 5 out of the 12 colonies harbored the desired plasmid, whichwas named “pMDGAT1-17” (FIG. 4C; SEQ ID NO:165).

“Control” vector pZUF-MOD-1 (SEQ ID NO:168) was prepared as follows.First, primers pzuf-mod1 (SEQ ID NO:166) and pzuf-mod2 (SEQ ID NO:167)were used to amplify a 252 bp “stuffer” DNA fragment using PDNR-LIB(ClonTech, Palo Alto, Calif.) as template. The amplified fragment waspurified with a Qiagen QiaQuick PCR purification kit, digested with NcoIand NotI using standard conditions, and then purified again with aQiaQuick PCR purification kit. This fragment was ligated into similarlydigested NcoI-/NotI-cut pZUF17 vector (SEQ ID NO:164; FIG. 10E) and theresulting ligation mixture was used to transform E. coli Top10 cells(Invitrogen). Plasmid DNA was purified from 4 resulting colonies, usinga Qiagen QiaPrep Spin Miniprep kit. The purified plasmids were digestedwith NcoI and NotI to confirm the presence of the ˜250 bp fragment. Theresulting plasmid was named “pZUF-MOD-1” (FIG. 4D; SEQ ID NO:168).

Y. lipolytica strain Y2067U (from Example 10, producing 14% EPA of totallipids) was transformed with pMDGAT1-17 and pZUF-MOD-1, respectively,according to the General Methods. Transformants were grown for 2 days insynthetic MM supplemented with amino acids, followed by 4 days in HGM.The fatty acid profile of two transformants containing pMDGAT1-17 andtwo transformants containing pZUF-MOD-1 are shown below in Table 18,based on GC analysis (as described in the General Methods). Fatty acidsare identified as 18:0, 18:1 (oleic acid), 18:2 (LA), GLA, DGLA, ARA,ETA and EPA; and the composition of each is presented as a % of thetotal fatty acids. TABLE 18 Lipid Content In Yarrowia Strain Y2067UEngineered To Overexpress M. alpina DGAT1 Total Fatty Acids Strain 18:018:1 18:2 GLA DGLA ARA ETA EPA Y2067U + pZUF-MOD-1 #1 1.31 6.92 12.0323.11 5.72 1.05 3.80 13.20 Y2067U + pZUF-MOD-1 #2 1.39 6.83 12.15 21.995.83 1.07 3.82 13.47 Y2067U + pMDGAT1-17 #1 0.89 7.13 10.87 24.88 5.821.19 3.97 14.09 Y2067U + pMDGAT1-17 #2 0.86 7.20 10.25 22.42 6.35 1.264.38 15.07As demonstrated above, expression of the M. alpina DGAT1 from plasmidpMDGAT1-17 increased the EPA concentration from ˜13.3% in the “control”strains to ˜14.1% (“Y2067U+pMDGAT1-17 #1”) and ˜15.1%(“Y2067U+pMDGAT1-17 #2”), respectively.

Example 16 Identification of DGAT1 Fungal Homologs

The present Example describes the use of the Yarrowia lipolytica andMortierella alpina DGAT1 sequences (SEQ ID NOs:13 and 17, respectively)to identify orthologous proteins in other fungi.

Orthologous DGAT1 fungal proteins were identified by conducting BLASTsearches for similarity to sequences contained in the BLAST “nr”database (comprising all non-redundant GenBank CDS translations,sequences derived from the 3-dimensional structure Brookhaven ProteinData Bank, the SWISS-PROT protein sequence database, EMBL and DDBJdatabases). The Yarrowia lipolytica and Mortierella alpina DGAT1sequences (SEQ ID NOs:13 and 17) were analyzed for similarity to allpublicly available DNA sequences contained in the “nr” database usingthe BLASTN algorithm provided by the National Center for BiotechnologyInformation (NCBI). The DNA sequences were translated in all readingframes and compared for similarity to all publicly available proteinsequences contained in the “nr” database, using the BLASTX algorithm(Gish, W. and States, D. J. Nature Genetics 3:266-272 (1993)) providedby the NCBI. These searches resulted in the identification of 4orthologous proteins (as shown below in Table 19). Table 19 additionallyshows the results of sequence comparisons between the Yarrowialipolytica DGAT1 sequence (SEQ ID NO:13) with each of the DGAT1 proteinsdisclosed herein, in terms of the observed “% Ident.” (defined as thepercentage of amino acids that are identical between the two proteins).TABLE 19 Comparison Of Yarrowia lipolytica DGAT1 To Orthologous DGAT1sFrom Fungi SEQ Abbre- % GenBank Accession No., ID Organism viation IdentReference and Annotation NO Mortierella Ma 32.4 — 18 alpina DGAT1Neurospora Nc 37.0 XP_322121; gi|32403016|ref| 19 crassa strain DAGAT1XP_322121.1| hypothetical OR74A protein; gi|28918105|gb| EAA27786.1|hypothetical protein Gibberella Fm 38.1 EAA77624; gi|42554781|gb| 20zeae PH-1 DAGAT1 EAA77624.1| hypothetical protein FG06688.1 MagnaportheMg 36.2 EAA52634; gi|38106308| 21 grisea 70-15 DAGAT1 gb|EAA52634.1|hypothetical protein MG05326.4 Aspergillus An 41.7 EAA57945; 22 nidulansDAGAT1 gi|40738755|gb|EAA57945.1| FGSC A4 hypothetical protein AN6159.2

Example 17 Identification of Universal and Fungal DGAT1 Motifs

The present Example describes the use of the DGAT1 sequences of thepresent invention, in conjunction with other known DGAT1 sequences, toidentify fungal and universal DGAT1 motifs.

To identify motifs (i.e., a set of amino acids conserved at specificpositions along an aligned sequence of evolutionarily related proteins)that are indicative of a DGAT1 protein, it was first necessary togenerate an alignment of DGAT1 sequences. For this, the following fungalsequences were used: SEQ ID NOs:14, 18, 19, 20, 21 and 22. Additionally,DGAT1 orthologs from 6 non-fungal sources were also included in thecomparative alignment: mouse (Mm DGAT1; GenBank Accession No. AF384160,corresponding to SEQ ID NO:169 herein); soy (Gm DGAT1; SEQ ID NO:16 ofUS20040088759A1, corresponding to SEQ ID NO:170 herein); Arabidopsis (AtDGAT1; SEQ ID NO:2 of US20040088759A1, corresponding to SEQ ID NO:171herein); rice (Os DGAT1; SEQ ID NO:14 of US20040088759A1, correspondingto SEQ ID NO:172 herein); wheat (Ta DGAT1; SEQ ID NO:22 ofUS20040088759A1, corresponding to SEQ ID NO:174 herein); and Perillafrutescens (Pf; GenBank Accession No. AF298815, corresponding to SEQ IDNO:173 herein).

Alignment was done using the Megalign program of DNASTAR using Clustal Wwith the following parameters: gap penalty=10, gap length penalty=0.2,delay divergent seqs (%)=30, DNA transition weight 10=0.5 and proteinweight matrix by Gonnet series. The results of this alignment are shownin FIGS. 8 a, 8 b, 8 c, 8 d, 8 e, 8 f, 8 g and 8 h. Based on analysis ofthe alignment, 8 motifs were identified as unique to fungal DGAT1sequences. Additionally, 7 motifs that were universally present in DGAT1sequences from plants, animals and fungi were also deduced. TABLE 20Fungal and Universal DGAT1 Motifs Fungal Motif Universal Motif AlignmentSequence and Sequence and Motif Position* SEQ ID NO SEQ ID NO # 1 97-104 (F/Y) xG F xN(L/I) xxGxxNxx (M/G) (SEQ ID NO:31) (SEQ ID NO:23)# 2 278-284 (P/Q) YPxN(I/V)T n/a (SEQ ID NO:24) # 3 334-340 QY A xPx(L/M) Q(Y/W)xxPxx (SEQ ID NO:25) (SEQ ID NO:32) # 4 364-374 KL(A/S)(T/S) x₁ S XX KL(A/S)xxxxxxWL (I/V)WL (SEQ ID NO:33) (wherein x₁ can notbe P) (SEQ ID NO:26) # 5 418-424 PV (Y/N)(Q/T/I) PVxxxxx (Y/F) (F/M)(K/R) (SEQ ID NO:34) (SEQ ID NO:27) # 6 415-424 WN(K/R/S)PV (Y/N) x₁WNxPVxxxxx (Y/F) (F/M) (K/R) (SEQ ID NO:35) (wherein x₁ can not be K)(SEQ ID NO:28) # 7 456-466 L x G xP T Hxx (I/Y)G xxxxPxxxxxx (SEQ IDNO:29) (SEQ ID NO:36) # 8 513-519 A(L/F)(L/M)Y( F /Y)X x(L/F)(L/M)Yxxx(A/H) (SEQ ID NO:37) (SEQ ID NO:30)*[Note:Alignment positions are with respect to that of the Yarrowialipolytica DGAT1, herein identified as SEQ ID NO:14. Those residuesshown in bold-type face and underlined are conserved only in fungalDGAT1 sequences.]These motifs, located at positions 97-104, 278-284, 334-340, 364-374,418-424, 415-424, 456-466 and 513-519 (wherein the alignment positionsare with respect to SEQ ID NO:14) in a sequence alignment of a family ofprotein homologues, have a high degree of conservation among DGAT1proteins; as such, it is expected that the amino acids residues locatedtherein are essential in the structure, the stability, or the activityof the protein. Based on the sequence conservation observed, one skilledin the art will know how to use the motifs provided as SEQ ID NOs:23-37as an identifier, or “signature”, to determine if a protein with a newlydetermined sequence belongs to the DGAT1 protein family describedherein.

1-4. (canceled)
 5. An isolated nucleic acid molecule comprising a firstnucleotide sequence encoding a diacylglycerol acyltransferase-1 enzymeof at least 526 amino acids that has at least 70% identity based on theBLASTP method of alignment when compared to a polypeptide having thesequence as set forth in SEQ ID NO:14; or a second nucleotide sequencecomprising the complement of the first nucleotide sequence.
 6. Anisolated nucleic acid molecule comprising a first nucleotide sequenceencoding a diacylglycerol acyltransferase-1 enzyme of at least 525 aminoacids that has at least 70% identity based on the BLASTP method ofalignment when compared to a polypeptide having the sequence as setforth in SEQ ID NO:18; or a second nucleotide sequence comprising thecomplement of the first nucleotide sequence.
 7. An isolated nucleic acidmolecule encoding an acyl-CoA:sterol-acyltransferase, selected from thegroup consisting of: (a) an isolated nucleic acid molecule encoding theamino acid sequence as set forth in SEQ ID NO:16; (b) an isolatednucleic acid molecule that hybridizes with (a) under the followinghybridization conditions: 0.1×SSC, 0.1% SDS, 65° C. and washed with2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1% SDS; or (c) an isolatednucleic acid molecule that is completely complementary to (a) or (b). 8.The isolated nucleic acid molecule of claim 7 as set forth in SEQ IDNO:15. 9-10. (canceled)
 11. An isolated nucleic acid molecule comprisinga first nucleotide sequence encoding an acyl-CoA:sterol-acyltransferaseenzyme of at least 543 amino acids that has at least 70% identity basedon the BLASTP method of alignment when compared to a polypeptide havingthe sequence as set forth in SEQ ID NO:16; or a second nucleotidesequence comprising the complement of the first nucleotide sequence.12-18. (canceled)
 19. A method of obtaining a nucleic acid moleculeencoding a diacylglycerol acyltransferase-1 enzyme comprising: a)probing a genomic library with a nucleic acid molecule selected from thegroup consisting of SEQ ID NO:13 and SEQ ID NO:17; b) identifying a DNAclone that hybridizes with the nucleic acid molecule of (a); and c)sequencing the genomic fragment that comprises the clone identified instep (b); wherein the sequenced genomic fragment encodes adiacylglycerol acyltransferase-1 enzyme.
 20. A method of obtaining anucleic acid molecule encoding a diacylglycerol acyltransferase-1 enzymecomprising: a) synthesizing at least one oligonucleotide primercorresponding to a portion of the sequence selected from the groupconsisting of SEQ ID NOs:13 and 17; and b) amplifying an insert presentin a cloning vector using the oligonucleotide primer of step (a);wherein the amplified insert encodes a portion of an amino acid sequenceencoding a diacylglycerol acyltransferase
 1. 21. The product of themethod of claims 19 or
 20. 22. An isolated nucleic acid moleculeencoding an amino acid motif selected from the group consisting of: a)SEQ ID NO:31; b) SEQ ID NO:32; c) SEQ ID NO:33; d) SEQ ID NO:34; e) SEQID NO:35; f) SEQ ID NO:36; g) SEQ ID NO:37; h) SEQ ID NO:23; i) SEQ IDNO:24; j) SEQ ID NO:25; k) SEQ ID NO:26; l) SEQ ID NO:27; m) SEQ IDNO:28; n) SEQ ID NO:29; and o) SEQ ID NO:30.
 23. An amino acid motifsequence selected from the group consisting of: a) SEQ ID NO:31; b) SEQID NO:32; c) SEQ ID NO:33; d) SEQ ID NO:34; e) SEQ ID NO:35; f) SEQ IDNO:36; g) SEQ ID NO:37; h) SEQ ID NO:23; i) SEQ ID NO:24; j) SEQ IDNO:25; k) SEQ ID NO:26; l) SEQ ID NO:27; m) SEQ ID NO:28; n) SEQ IDNO:29; and o) SEQ ID NO:30.
 24. A method of increasing triacylglycerolcontent in a transformed host cell comprising: (a) providing atransformed host cell comprising: (i) at least one gene encoding adiacylglycerol acyltransferase-1 enzyme having the amino acid sequenceselected from the group consisting of SEQ ID NOs:14, 18, 19, 20, 21 and22 under the control of suitable regulatory sequences; and, (ii) asource of fatty acids; (b) growing the cell of step (a) under conditionswhereby the at least one gene encoding a diacylglycerol acyltransferase1 enzyme is expressed, resulting in the transfer of the fatty acids totriacylglycerol; and (c) optionally recovering the triacylglycerol ofstep (b).
 25. A method of increasing the ω-3 or ω-6 fatty acid contentof triacylglycerols in a transformed host cell comprising: (a) providinga transformed host cell comprising: (i) genes encoding a functionalω-3/ω-6 fatty acid biosynthetic pathway; (ii) at least one gene encodinga diacylglycerol acyltransferase 1 enzyme having the amino acid sequenceselected from the group consisting of SEQ ID NOs:14, 18, 19, 20, 21 and22 under the control of suitable regulatory sequences; (b) growing thecell of step (a) under conditions whereby the genes of (i) and (ii) areexpressed, resulting in the production of at least one ω-3 or ω-6 fattyacid and its transfer to triacylglycerol; and (c) optionally recoveringthe triacylglycerol of step (b).
 26. A method of increasingtriacylglycerol content in a transformed host cell comprising: (a)providing a transformed host cell comprising: (i) at least one geneencoding a diacylglycerol acyltransferase-1 enzyme comprising all of theamino acid motifs as set forth in: 1) SEQ ID NO:31; 2) SEQ ID NO:32; 3)SEQ ID NO:33; 4) SEQ ID NO:34; 5) SEQ ID NO:35; 6) SEQ ID NO:36; and 7)SEQ ID NO:37; under the control of suitable regulatory sequences; and,(ii) a source of fatty acids; (b) growing the cell of step (a) underconditions whereby the at least one gene encoding a diacylglycerolacyltransferase-1 enzyme is expressed, resulting in the transfer of thefatty acids to triacylglycerol; and (c) optionally recovering thetriacylglycerol of step (b).
 27. A method of increasing triacylglycerolcontent in a transformed host cell comprising: (a) providing atransformed host cell comprising: (i) at least one gene encoding adiacylglycerol acyltransferase 1 enzyme comprising all of the amino acidmotifs as set forth in: 1) SEQ ID NO:23; 2) SEQ ID NO:24; 3) SEQ IDNO:25; 4) SEQ ID NO:26; 5) SEQ ID NO:27; 6) SEQ ID NO:28; 7) SEQ IDNO:29; and 8) SEQ ID NO:30; under the control of suitable regulatorysequences; and, (ii) a source of fatty acids; (b) growing the cell ofstep (a) under conditions whereby the at least one gene encoding adiacylglycerol acyltransferase-1 enzyme is expressed, resulting in thetransfer of the fatty acids to triacylglycerol; and (c) optionallyrecovering the triacylglycerol of step (b).
 28. A method of increasingthe ω-3 or ω-6 fatty acid content of triacylglycerols in a transformedhost cell comprising: (a) providing a transformed host cell comprising:(i) genes encoding a functional ω-3/ω-6 fatty acid biosynthetic pathway;and (ii) at least one gene encoding a diacylglycerol acyltransferase 1enzyme comprising all of the amino acid motifs as set forth in: 1) SEQID NO:31; 2) SEQ ID NO:32; 3) SEQ ID NO:33; 4) SEQ ID NO:34; 5) SEQ IDNO:35; 6) SEQ ID NO:36; and 7) SEQ ID NO:37; under the control ofsuitable regulatory sequences; (b) growing the cell of step (a) underconditions whereby the genes of (i) and (ii) are expressed, resulting inthe production of at least one ω-3 or ω-6 fatty acid and its transfer totriacylglycerol; and (c) optionally recovering the triacylglycerol ofstep (b).
 29. A method of increasing the ω-3 or ω-6 fatty acid contentof triacylglycerols in a transformed host cell comprising: (a) providinga transformed host cell comprising: (i) genes encoding a functionalω-3/ω-6 fatty acid biosynthetic pathway; and (ii) at least one geneencoding a diacylglycerol acyltransferase 1 enzyme comprising all of theamino acid motifs as set forth in: 1) SEQ ID NO:23; 2) SEQ ID NO:24; 3)SEQ ID NO:25; 4) SEQ ID NO:26; 5) SEQ ID NO:27; 6) SEQ ID NO:28; 7) SEQID NO:29; and 8) SEQ ID NO:30; under the control of suitable regulatorysequences; (b) growing the cell of step (a) under conditions whereby thegenes of (i) and (ii) are expressed, resulting in the production of atleast one ω-3 or ω-6 fatty acid and its transfer to triacylglycerol; and(c) optionally recovering the triacylglycerol of step (b).
 30. A methodaccording to claim 25, 28 or 29, wherein the genes encoding a functionalω-3/ω-6 fatty acid biosynthetic pathway are selected from the groupconsisting of desaturases and elongases.
 31. A method according to claim30, wherein the desaturase is selected from the group consisting of: Δ9desaturase, Δ12 desaturase, Δ6 desaturase, Δ5 desaturase, Δ17desaturase, a Δ8 desaturase, Δ15 desaturase and Δ4 desaturase.
 32. Amethod according to any of claims 24-29, wherein the host cell isselected from the group consisting of algae, bacteria, molds, fungi andyeasts.
 33. A method according to claim 32, wherein the host cell is anoleaginous yeast.
 34. A method according to claim 33 wherein theoleaginous yeast is a member of a genus selected from the group ofconsisting of Yarrowia, Candida, Rhodotorula, Rhodosporidium,Cryptococcus, Trichosporon and Lipomyces.
 35. A method according toclaim 34, wherein the oleaginous yeast is Yarrowia lipolytica.
 36. Amethod according to claim 35, wherein the Yarrowia lipolytica is astrain selected from the group consisting of Yarrowia lipolytica ATCC#20362, Yarrowia lipolytica ATCC #8862, Yarrowia lipolytica ATCC #18944,Yarrowia lipolytica ATCC #76982, Yarrowia lipolytica ATCC #90812 andYarrowia lipolytica LGAM S(7)1.
 37. A method according to any one ofclaims 24, 25, 26, 27, 28 or 29, wherein the fatty acid is selected fromthe group consisting of: stearate, oleic acid, linoleic acid, γ-linoleicacid, dihomo-γ-linoleic acid, arachidonic acid, α-linoleic acid,stearidonic acid, eicosatetraenoic acid, eicosapentaenoic acid,docosapentaenoic acid, eicosadienoic acid and eicosatrienoic acid.
 38. Amethod for the identification of a polypeptide having diacylglycerolacyltransferase-1 activity comprising: a) obtaining the amino acidsequence of a polypeptide suspected of having diacylglycerolacyltransferase-1 activity; and, b) identifying, in the amino acidsequence of the polypeptide of step (a), the presence of all of theamino acid motif sequences as set forth in: 1) SEQ ID NO:31; 2) SEQ IDNO:32; 3) SEQ ID NO:33; 4) SEQ ID NO:34; 5) SEQ ID NO:35; 6) SEQ IDNO:36; and 7) SEQ ID NO:37; wherein the presence of all of the motifsequences of step (a) in the polypeptide is indicative of diacylglycerolacyltransferase-1 activity.
 39. A method for the identification of afungal polypeptide having diacylglycerol acyltransferase-1 activitycomprising: a) obtaining the amino acid sequence of a fungal polypeptidesuspected of having diacylglycerol acyltransferase-1 activity; and, b)identifying, in the amino acid sequence of the polypeptide of step (a),the presence of all of the amino acid motif sequences as set forthin: 1) SEQ ID NO:23; 2) SEQ ID NO:24; 3) SEQ ID NO:25; 4) SEQ ID NO:26;5) SEQ ID NO:27; 6) SEQ ID NO:28; 7) SEQ ID NO:29; and 8) SEQ ID NO:30;wherein the presence of all of the motif sequences of step (a) in thepolypeptide is indicative of diacylglycerol acyltransferase-1 activity.